管道倒流防止器是由__组成. ( )
A.管道进口止回阀、自动 泄水阀和出口止回阀;
B.管道进口止回阀、自动泄水阀;
C.管道进口止回阀、空气隔断短管和出口止回阀;
D.?管道进口止回阀和出口止回阀.