There _____________ a pen and two pencils in the box. [ ]
A. are
B. is
C. has
D. have
答案:B
遗传因子组成AaBbCCDDee与AABbCcDDEe交配,在子代中,纯合子的比例是[ ]
A.1/4
B.1/8
C.1/16
D.1/32
找转子的剩余静不平衡时,在叶轮上画一个配重圆,将配重的圆周分为()等份。
A.2
B.4
C.8
D.16