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