_______for  the  captain ,the  ship  would  have  sunk  with  all  on  board .

A． Hadn’t it been

B．Had it been not

C．Not had it been

D．Had it not been

采用闭合或附和水准线路测量时，高差闭合差的容许值为（）。

A、±2√nmm

B、-2√nmm

C、±√2nmm

D、-√2nmm