In the 1960s, marine biologists believed that dolphins used echolocation primarily for _____ recent studies have revealed that these marine...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• In the 1960s,
  • marine biologists believed
    • that dolphins used echolocation primarily for navigation [?]
• recent studies have revealed
  • that these marine mammals also employ this biological sonar system for complex social interactions.

Understanding the Meaning

Let's start reading from the beginning:

• In the 1960s, marine biologists believed that dolphins used echolocation primarily for navigation...

This is where we have the blank. Let's look at the choices:

• Some have just 'navigation' with nothing after
• Some add a comma
• Some add 'but'
• Some add both comma and 'but'

To see what works here, let's read the rest of the sentence and understand what it's saying!

The sentence continues:

• 'recent studies have revealed that these marine mammals also employ this biological sonar system for complex social interactions.'

Now let's understand what this complete sentence is telling us:

• First part: 'marine biologists believed that dolphins used echolocation primarily for navigation'
  • This is telling us what scientists thought in the 1960s
  • This is a complete thought - it could stand alone as its own sentence
• Second part: 'recent studies have revealed that these marine mammals also employ this biological sonar system for complex social interactions'
  • This is telling us what recent research has shown
  • Notice the word 'also' - this signals they're adding NEW information beyond just navigation
  • This is also a complete thought - it could stand alone too

What do we notice about the structure here?

• We have two complete thoughts that need to be connected
  • Each part has its own subject and verb
  • Each part could be a sentence by itself
• The relationship between them is contrasting/expanding:
  • The old belief: primarily for navigation
  • The new discovery: ALSO for social interactions (something beyond what was originally thought)

When we connect two complete thoughts like this, we need:

• A coordinating conjunction (a connecting word like 'but') to show the relationship
• A comma before that conjunction

So we need: navigation, but

The correct answer is C.

GRAMMAR CONCEPT APPLIED

Connecting Two Complete Thoughts with Coordinating Conjunctions

When you want to join two complete thoughts (each with its own subject and verb that could stand alone as sentences) into one sentence, you need to use both a comma and a coordinating conjunction (these are called independent clauses in grammar terms):

Pattern: [Complete thought], + [coordinating conjunction] + [complete thought]

The coordinating conjunctions are: for, and, nor, but, or, yet, so (FANBOYS)

Examples:

• Two separate sentences: The experiment succeeded. The results surprised everyone.
• Properly connected: The experiment succeeded, but the results surprised everyone.
  • First complete thought: 'The experiment succeeded'
  • Comma + coordinating conjunction: ', but'
  • Second complete thought: 'the results surprised everyone'

• Two separate sentences: Marine biologists believed dolphins used echolocation for navigation. Recent studies revealed they also use it for social interactions.
• Properly connected: Marine biologists believed dolphins used echolocation for navigation, but recent studies revealed they also use it for social interactions.

Why both comma AND conjunction are needed:

• The comma alone = comma splice error (run-on)
• The conjunction alone = missing required comma
• Both together = proper connection

Choosing the right conjunction:

• 'But' shows contrast or adds unexpected information
• In this question, 'but' works because the new studies reveal something BEYOND what was originally believed