By 1999, astronomers had discovered 17 nearby stars

By 1999, astronomers had discovered 17 nearby stars that are orbited by planets about the size of Jupiter.

1 Explanation

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Adam

We need the past perfect to denote an event that occurred before a specified time in the past, so eliminate (D) and (E). (B) and (C) are unnecessarily wordy. (A) presents the information in the most concise manner.

Sep 1, 2017 • Comment

xiaoyang wu

I am struggling between A and E.
"an event that occurred before a specified time in the past.“
How to confirm above time frame?
Why we can assure that the stated sentence is on the same time frame?

Apr 25, 2020 • Reply

Adam Lyons, Magoosh Tutor

Today is after 1999. So 1999 is in the past. According to the sentence, though, astronomers had discovered 17 nearby stars *before* 1999. So we have an action that took place before a specified time in the past. The discovering took place before 1999. So that's why we need the past perfect.

May 10, 2020 • Reply