If a is positive and b is negative, which of the following must be negative?

A、<img width="35" height="17" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/84fe2d8d-4a29-40a3-90da-06c41c024503.png">

B、<img width="40" height="26" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/d4c87388-b907-4de6-a9b9-ea5ef38687c2.png">

C、<img width="40" height="26" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/4715ef5e-d988-4608-a31d-15fbcc84757c.png">

D、<img width="26" height="26" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/e268db09-ee9f-4ce6-9319-099b6795bbb5.png">

E、<img width="28" height="26" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/08586172-1b77-49a2-a0db-07d6f8185f0b.png">

A、<img width="35" height="18" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/24ace65d-eb95-4d5b-aa2f-4b8a883e7060.png">

B、<img width="40" height="27" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/ec6e06d1-ed95-4833-a496-45b9fe9bf9f3.png">

C、<img width="40" height="27" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/86a78478-a4f7-4c25-a623-581faab06fd3.png">

D、<img width="27" height="27" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/5dc77703-bb82-4387-b7b0-adf64e027e49.png">

E、<img width="28" height="27" src="https://static.tiw.cn//s/tiw/p3/UpLoadImage/2016-09-19/00488909-f083-4642-9de8-3ec06d2ae272.png">