A:dow
B:out
C:u
D:off
A:Hi, Li Ming.
B:Hi, Mrs Li.
A:What are you doing?
B:I'm reading a story book. What are you doing?
A:I'm writing a letter. May I look at your story book?
B:Certainly. Here you are.
A:Thank you.
如图所示,a、b、c、d是距凸透镜不同距离的四个点,F为焦点。下列几种光学仪器的成像原理与物体在不同点时的成像情况相对应,下列说法正确的是( )
下列式子中,是方程的是( )