PCR即聚合酶链反应，由变性、退火、引物延伸三个基本反应组成的循环性过程。（）
参考答案：对
Hearing his father was seriously ill, ______. [ ]
A. he burst into tears
B. his eyes were filled with tears
C. his face lost its colour
D. tears came to his eyes
下述定义变量指令正确的是( )。
A．X DW'ABCD'
B．X DB 'A','B'，'C'，'D'
C．X DD'ABCD'
D．X DQ'ABCD'
