A when adverb modifying a verb is the best way to describe “later” in the following sentence. Hence, option A is correct.
Adverb is the that modify or describe the adjective, it shows the degree, manner, place, or time of the sentence.
In the sentence "at almost three o'clock on a very hot day," the adverbs "almost" and "very" are used.
Thus, option A is correct.
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