A survey of the opinions of experts _____ that three hours of outdoor exercise a week _____  good for

one's health. [ ]

A. show; are

B. shows; is

C. show; is

D. shows; are

阀门的阀杆与阀盖间是依靠（）来密封的，填料的选用应根据（）和（）的不同来确定。