为了节约用电和使用方便，我们教室的六盏灯要用三个开关控制，一个开关控制两盏灯，请设计符合要求的电路，在方框中画出电路图．
以下说法正确的是[ ]
A．扩散现象只能发生在气体与气体间
B．扩散现象只能发生在液体与液体间
C．扩散现象只能发生在固体与固体间
D．任何物质间都可发生相互扩散
I wrote a letter to John for his help during my high school years，without which I ________ the college entrance exam.
A．would fail
B．would have failed
C．should fail
D．must have failed
