三级预防是指_____、_____、_____。
参考答案:病因预防;临床前预防;临床预防
物体放在凸透镜前,在另一侧距透镜15cm的光屏上成了一个清晰缩小的像,则此凸透镜的焦距可能为( )
A.6cm
B.7.5cm
C.10cm
D.20cm
Mom, I’m old enough to wash ________ own clothes and ________. You can have a rest after work.
A.my; your
B.my; yours
C.your; my
D.yours; mine