I hope to visit Hawaii some day. Is it a place _______ the weather is always warm?
A:what
B:when
C:where
D:which
以下天体与我们的视觉对应正确的是( )
一物体放在凸透镜前18 cm处,在透镜另一侧20 cm处成一清晰像,则此透镜的焦距 ( )