单选题 -- Can you go shopping with us this afternoon?
-- Sorry, I can't. I ______ take care of my little sister because she is ill.

【正确答案】 A
【答案解析】考查情态动词。have to指“必须”,may表示“可以,可能”,would表示意愿和(过去)将来、或者表示委婉语气。由空格前的can't可知应选[A] 。