设向量a，b满足：|a|=1，|b|=2 ，a·（a+b）=0 ，则a与b的夹角是（ ）。
120°
I don’t know whether you happen_______, but I’m going to study in the U.S.A. this September.
A. to be heard B. to be hearing
C. to hear D. to have heard
毒性药品每次配料必须做到
A．专人复核无误B．一人以上复核无误C．二人复核无误D．二人以上复核无误E．三人以上复核无误
