Sentence Improvement - Verbal and Reasoning

Directions to SolveIn questions given below, a part of the sentence isitalicised and underlined. Below are given alternatives to the italicised part which may improve the sentence. Choose the correct alternative. In case no improvement is needed, option 'D' is the answer.
Q1:

His father won't be able to leave for Varnasi until they have arrived.

A until they arrive

B until they will have arrived

C until they will arrive

D No improvement

ANS:A - until they arrive

No answer description is available.