两点测验和（）是基因定位所采用的主要方法。
参考答案：三点测验
–A boy was killed by the tiger in the zoo.
–He was believed ______ into the cage while feeding the tiger without permission.
A．to fall
B．to be falling
C．to have fallen
D．having fallen
单轨吊机车“开锅”原因及排除方法？
