By the time the research team ______ the excavation site in northern Chile, erosion from seasonal floods had already exposed...

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

• By the time the research team [reached/will reach/reaches/is reaching] the excavation site in northern Chile,
• erosion from seasonal floods
• had already exposed several fossilized vertebrae,
• revealing what would later be identified as a previously unknown species of herbivorous dinosaur.

Understanding the Meaning

Let's start reading from the beginning:

"By the time the research team ______ the excavation site in northern Chile..."

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

• will reach (future tense)
• reached (simple past)
• reaches (simple present)
• is reaching (present continuous)

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

The sentence continues:
"erosion from seasonal floods had already exposed several fossilized vertebrae"

Now let's understand what this is telling us:

• "Had already exposed"
  • This is telling us about something that happened in the past
  • The word "had" + "exposed" indicates this erosion happened BEFORE another past event
  • That's what this verb form does - it shows one past action happened before another past action
• "Already"
  • This emphasizes the timing - the erosion had finished doing its work by the time something else happened

So what's the time picture here?

• The erosion exposed the vertebrae (this happened first)
• The research team arrived at the site (this happened second)
• Both events are in the past
• But the erosion was already complete when the team got there

What do we notice about the structure?

• "Had already exposed" is pointing backward in time to something that happened before another past moment
• The team's arrival is that reference point in the past
• So we need simple past tense for when the team arrived: reached

The sentence is saying: By the time the team arrived (reached) in the past, the erosion had already finished its work (had exposed) - even earlier in the past.

The rest confirms this timeline:

• "revealing what would later be identified as a previously unknown species of herbivorous dinosaur"
  • "Would later be identified" - this continues the past time frame, talking about what happened after, from that past perspective

So we need: B. reached

GRAMMAR CONCEPT APPLIED

Verb Tense Sequencing: Using Simple Past with Past Perfect

When you're describing two actions that both happened in the past, but one happened BEFORE the other, English uses a specific tense combination to show that sequence:

The Pattern:

• Earlier past action - past perfect (had + past participle)
• Later past action (reference point) - simple past

Example 1:
By the time we arrived at the theater, the movie had already started.

• "arrived" = simple past (the reference point)
• "had started" = past perfect (happened before we arrived)

Example 2:
She realized she had left her keys at home.

• "realized" = simple past (the reference point)
• "had left" = past perfect (the leaving happened before the realizing)

In our question:
By the time the research team reached (simple past - reference point) the excavation site, erosion had already exposed (past perfect - happened earlier) several fossilized vertebrae.

The signal phrase "by the time" often introduces this pattern - it's setting up a comparison between when two things happened, with one being complete before the other began or occurred.