There are three different issues here. The first is that the verb to be chosen must be singular because the subject is RATIO. The second is the choice of either IS or HAS BEEN. Clearly, you must choose HAS BEEN because the sentence is talking about something that has been the case SINCE 1960 until the present. Thus, IS would not be acceptable. HAS BEEN, on the other hand, refers to an action extending from a certain past time until the present.

The third issue is whether, as you say, SINCE always mandates in the GMAT that the present perfect be used. The answer is no - but bear in mind that when the present perfect is used, SINCE is very commonly used, because the present perfect often refers to actions that have started at some past time and have continued until the present.

However, look at this sentence: The shop assistant claimed that she had been standing behind the counter since 10 a.m. and that not a single customer had approached her. Here, you have SINCE + past perfect continuous. (Reported speech)

Here is a different case: Unidentified flying objects (UFOs), or flying saucers, are believed to have visited Earth on a number of occasions since they were first sighted in Classical Antiquity. Here, you have a passive verb (ARE BELIEVED) followed not by the present perfect but by a perfect infinitive (TO HAVE VISITED) and then SINCE .

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