利用对称性求解图示结构内力时的位移法未知数的个数为（）。
A.2
B.3
C.4
D.5
参考答案：C
解析：考查位移法基本概念
The big fire in Shanghai, believed to ______ due to the burning of construction materials, caused 58 deaths and destroyed all the belongings of the people there.
A．have occurred
B．occur
C．have been occurring
D．be occurring
抢救伤员时，如发现心脏停止跳动，应立即（）抢救。
A.掐“人中”
B.实施心肺复苏
C.送往医院
D.拍打胸部
