造成义齿摘戴困难的原因是
A．支托不就位
B．基托进入组织倒凹
C．基托与粘膜组织不密合
D．卡环不贴合
E．人工牙咬合高
参考答案：B
若向量a、b满足|a|=|b|=1，a与b的夹角为120°，则a·a+a·b=（ ）。
Lots of waste has been dumped into the river，which has led to________.
A．lots of fish having been killed
B．kill lots of fish
C．lots of fish being killed
D．lots of fish to kill
