The young pioneers kept on working in the fields ________ they felt tired and thirsty.[ ]

A. but  

B. or

C. even though  

D. since

噪声所致听力曲线图上的V形下陷常发生在（）

A.500～2000Hz

B.1000～3000Hz

C.2000～5000Hz

D.3000～6000Hz

E.4000～8000Hz