热力管网的平面布置为环状布置的优点为（）。
A．钢材耗用量小
B．投资小
C．具有供热的后备性能
D．比较简单
参考答案：C
Distinguished guests and friends, welcome to our school, _____the ceremony of the 50th Anniversary this morning are our alumni （校友） from home and abroad.
A．Attend
B．To attend
C．Attending
D．Having attended
若已知引力常量G、地球半径R和地球表面重力加速度g，地球质量可表示为______．
