两个质量均为的完全相同的金属球a和，其质量分布均匀，将它们固定于绝缘支座上，两球心间的距离为球半径的3倍。若使它们带上等量异种电荷，所带电荷量的绝对值均为Q，那么a、b两球之间的万有引力F_引和库仑力F_库的表达式正确的是

A．

B．

C．

D．

--- What you’ve said sounds _____, but can we really solve the problem in that way?

--- It is worth a try.

A．absurd

B．awesome

C．shallow

D．reasonable