我国有“水能宝库”之称的河流是( )
A:黑龙江
B:黄河
C:长江
D:珠江
矫正近视眼,应该配戴镜片是凹透镜的眼镜,凹透镜对光线 ____作用.
(Ⅰ)求角A的大小;
(Ⅱ)若a=1, AB→•AC→=3 ,求b+c的值.
He will come to understand your efforts sooner or later. It's just a matter of_______.