-I will go to the countryside to see my grandmother this afternoon, so I can't go swimming with you.
-___________.
A:It's my pleasure
B:You are welcome
C:What a pity
D:Sure, I am
读下面“世界某地区简图”,回答下列各题。
如图所示的苹果树上结有三个质量相等的苹果,其中重力势能最大的苹果是( )