系统性因素引起的质量变异，一般属于（）。
A．正常变异
B．非正常变异
C．随机变异
D．有规律变异
参考答案：B
He ____ a teacher of computer. [ ]
A. is
B. am
C. are
_______ three nice _______ of my family.
A．Here are; photos
B．These are; photo
C．Here is; photo
D．This is; photoes
