Every means _____ tried but without much result. [ ]
a. has been
b. have been
c. are
d. is
a
You need some facts _____ to base your theory.
A.where
B.on which
C.in place where
D.in the place
小红在探究平面镜成像规律时,将一支蜡烛竖直放在一块很薄的透明玻璃前,蜡烛距离玻璃15cm,点燃蜡烛后,小红观察到了玻璃中蜡烛的像,这个像是由于光的______现象形成的,蜡烛的像到玻璃的距离为______ cm.