单项选择题
A.adjusting a nut
B.a pre-tensioned spring
C.any spring
D.upstream pressure
单项选择题 Most marine biological sewage treatment plants consist of all the following statements except （）.
单项选择题 To prevent accidents from occurring, special valve chests are used in such cases in which interchangeable blanks and connecting passages are incorporatedThe bold and italic word “which” means （）.
单项选择题 A check valve is another name for （）.