调节阀前后压差较小，要求泄漏量小，一般可选用（）。
A、单座阀
B、蝶阀
C、隔膜阀
D、偏心旋转阀
参考答案：A
Simon _____ the photos when I came to visit him. [ ]
A. enjoyed
B. is enjoying
C. was enjoying
D. has enjoyed
人的知觉是
A．多种感觉的叠加B．过去经验的反映C．脑对事物整体的反映D．思维的结果E．对知识认识和了解
