The development of industry has been ________ gradual process throughout ______ human existence from stone tools to modern technology.

A．不填；the

B．the; a

C．a; 不填

D．a; a

天文学家新发现太阳系外有一颗行星．这颗行星的体积是地球的M倍，质量是地球的N倍．已知某一近地卫星绕地球运动的周期为T，引力常量为G，则该行星的平均密度为                                        （   ）

A．

B．

C．

D．