He has been waiting for this letter for days, and at it has come.

Đáp án A.

A. at (long) last = finally. Ex: At last, we got home.

B. in the end: cuối cùng. Ex: He applied for different positions and in the end worked as an assisstant.

C. at present = D. at the moment = now.

Ex: She’s out at present! Can you leave a message?