Idon'tknowwhoinwented_______cellphone,butIthinkit's_______mostusefulinvention.A.the;theB.the;aC.a;theD.a;a为什么选择B?

　　Idon'tknowwhoinwented_______cellphone,butIthinkit's_______mostusefulinvention.

　　A.the;the

　　B.the;a

　　C.a;the

　　D.a;a

　　为什么选择B?