下列工艺原理不属于化学变化的是[ ]
A．煤干馏
B．石油分馏
C．石油的裂解
D．蔗糖的水解
答案：B
—You look unhappy. ?
—I have so much homework to do that I couldn't play soccer. [ ]
A. Do you feel sad
B. Are you ill
C. What's the'matter
串行码变换为并行码，码的传送速率（）。
A.不变
B.变慢
C.变快
D.不定
