In statement given below, a part of the sentence is enclosed in quotation marks. Below are given alternatives to the quotes part which may improve the sentence. Choose the correct alternative. In case no improvement is needed, option 'D' is the answer.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