—Oh, really? I ____________. I _____________ go and visit him.
A:didn't know; am going to
B:hadn't known; would
C:haven't known; will
D:didn't know; will
一个焦距为30cm的凸透镜,在它的主光轴上离凸透镜20cm处,放置一个物体,则该物体经过凸透镜所成的像是 ()
玻璃中加入二氧化钒可以有选择地吸收和反射色光,二氧化钒(VO2)中钒元素的化合价为( )