Fill in the blank with a suitable preposition.
The building will be ready _______ exactly one month from now.
The prepositions ‘in’ and ‘within’ are often confused with one another. ‘In’ is used to denote the end of a period of time in the future. Whereas ‘within’ is used to denote a time before the end of the period, i.e. the action can take place anytime inside a period. In the given context, the building will be ready only at the end of the period, which is clearly stated through the word ‘exactly’ and no sooner than that, hence option A is correct and option B is incorrect. The preposition ‘at’ is used to speak about an exact time or place which is irrelevant here, so option C is incorrect. Similarly ‘on’ is used before a day or date only, therefore option D is incorrect.