必须保障矿井双回路电源线路、井下供电不少于（），严禁擅自更改继电保护整定值。
A、三回路
B、两回路
C、单回路
D、备用电源
参考答案：B
Large quantities of fish _______ in the river in the past few days.
A．were caught
B．was caught
C．has been caught
D．have been caught
开放式基金的申购费率不得超过申购金额的（）。
A、2%
B、3%
C、4%
D、5%
