专门用于制备单链DNA的PCR技术是（）
A、对称PCR
B、反向PCR
C、RT－PCR
D、不对称PCR
E、嵌套PCR
参考答案：D
_____ his shoes under the bed on the floor?[ ]
A. Are
B. Is
C. Has
D. She's
金字塔型
