B is incorrect, unless Kathy is under 10.
C and D are definitely correct.
A is OK, but is not a great choice. When we say, "You can say that again", we mean "I wholeheartedly agree with you". Sarah �s statement just doesn �t seem to be strong enough to warrant such a response. This is totally subjective, though, and Kathy �s opinion of the importance of Sarah �s statement might be different from mine.
Anyway, I wouldn�t use it in this situation, but it is definitely correct, and some people might strongly disagree with me.
If Kathy is a 3rd-grade teacher whose students can �t stay awake in class, that is just what she would say.
Oh, the short answer is, "Yes, it�s fine".
Bruce