银行汇票的实际结算金额不得更改， 更改实际结算金额的银行汇票无效。( )
参考答案：对
They began to think about what use could be _______ such materials.
A．made up
B．made from
C．made of
D．made out
最基本的随机抽样方法是（）。
A.分层抽样
B.简单随机抽样
C.系统抽样
D.整群抽样
