?Can you go swimming with us this afternoon?

?Sorry, I can't. I ________ take care of my little sister at home because my mother is ill.

A. can

B. may

C. would

D. have to

--Can you go swimming with us this afternoon?

--Sorry, I can't. I ________ take care of my little sister at home because my mother is ill.

[　　]

--Can you go swimming with us this afternoon?

--Sorry, I can't. I ________ take care of my little sister at home because my mother is ill.

[　　]

--Can you go swimming with us this afternoon?

--Sorry, I can't. I ________ take care of my little sister at home because my mother is ill.

[　　]