下列关于地球大小的描述正确的是( )
A:平均半径约4万千米
B:赤道周长6371千米
C:表面积约5.1亿平方千米
D:表面积约5.2亿平方千米
Mary's grandpa is always ______ in the mountain village. He tells her he feels very______. So she's going to visit him next week.