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.
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?
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.
1 Explanation