Both answers are grammatical. I suppose (b) is semantically more likely. "have come" is present perfect; "must have reached" is a modal present perfect.
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GPY; "must have reached" is a modal present perfect.It's a modal perfect form, but it is not a modal present perfect. There is no present-tense verb in the construction. In any case, modal forms are, almost by definition, outside the tense system (unless you consider could, might, should and would to be the past-tense forms of can, may, shal
fivejedjonIt's a modal perfect form, but it is not a modal present perfect.Stole my thoughts.
GPYWhile I agree that "modal present perfect" is not a very common term (though not unknown), it is in this instance descriptively correct in my opinion, since "they must have reached" has the same tense intent as "they have reached", just with modal colouration.Tense intent?