—You seem a little blue today. What.s the matter?

—_____________________.

选择一项：
A. It.s been a difficult day.
B. It doesn.t matter.
C. I am a little sad.

A. It's been a difficult day. 解析：本题考核“表达情感”情境下的交际用语。第一说话人认为对方今天看上去心情不好，询问其原因。因此对方应该回答造成心情不好的原因即可，答语应用It's been a difficult day. （这是艰难的1天。）来回应，所以选选项“It's been a difficult day.”。