塑性围岩
参考答案：
包括各种软弱的层状结构岩体（如页岩、泥岩和粘土岩等）和散体结构岩体。
Leonardo da Vinci (1452-1519) ____ birds kept in cages in order to have the pleasure of setting them free.
A．is said to be buying
B．is said to have bought
C．had said to buy
D．has sad to have bought
简述委托合同的特征。
