tense question

Because the start of the conference happened first. In order to indicate this clearly, you need different tenses to show the difference in time.

thank you for your reply.
But isn't it possible that the two events(the arrival of delegates and the start of the conference) happened at the same time?

The answer is B.

No, because the sentence starts

By the time the delegates arrived at the convention center

"By the time" means "before"

By the time I arrived at the station, the train had left.

By the time I get to Phoenix, she'll be rising.....
And she'll find that note that I left hanging on the door.
And she'll laugh when she reads the part
That says: "I'm leaving"
Cause I've left that girl so many times before....

(A well known song recorded by Glenn Campbell and many others)

If the arrival of the delegates, and the start of the conference, happened at the same time, we would write something like "At the time the delegates arrived at the convention center, the international conference on climate started"

then how would you explain a sentence like " by the time he got there, the train was long gone. " ?? since this sentence means that the train was gone before he arrived, I guess the 'past tense-started' in the following question also can be the answer, indicating that 'the start of the meeting' happened first.... ??

By the time the delegates arrived at the convention center, the international conference on climate _________.
(A) started (B) had started (C) starts (D) will start

Though they look similar, these two sentences are subtly different from each other. Both of them describe events that occured prior to other events, but the prior event in the first sentence (i.e. "had started") has a definite stopping point while the prior event in the second sentence (i.e. "was long gone") continues.

"the international conference on climate had started"
This is the first event to happen in the first sentence, and it ends before the second event. That is to say, the act of "starting" happens only once--the conference has only one starting point. The conference started before he got there, but it was not still in the process of starting when he did get there.


"the train was long gone"
This is the first event to happen in the second sentence, but it does not end by the time the second event ("By the time he got there") occurs. Notice that in this clause, the predicate is an adjectival description--i.e. the sentence is not telling us what the train did, the sentence is attributing a property to the train and telling us what the train was. This property continues to be true by the time the second event occurs: the train was gone before he got there, and the train is still gone when he does get there.

Of course, it would be perfectly possible to rewrite the second sentence such that the prior event is an action rather than a property: "By the time he got there, the train had left." In the case, the sentence is now similar in structure to the first sentence. In fact, I suspect this version of the sentence would be stylistically preferred in formal writing, though it's a close call.