— Did you find the missing couple in the mountain yesterday?

— No, but we ____ to get in touch with them ever since. [ ]

A. have tried

B. have been trying

C. had tried

D. had been trying

某同学想用以下器材组装一只欧姆计，并测量一只约几千欧电阻的阻值．

A 电流表，满偏电流为1mA，内阻为20Ω

B 电流表，满偏电流为0.6A，内阻为5Ω

C 电动势15V，内阻5Ω的直流电源

D 电动势3V，内阻3Ω的直流电源

E 最大阻值为5000Ω的滑动变阻器

F 最大阻值为100Ω的滑动变阻器

（1）以上器材应选用______（填字母）

（2）欧姆计调零后，滑动变阻器被接入部分的阻值为______Ω

（3）若用此欧姆计测量电阻，发现指针指在满偏电流的三分之一处，则此电阻的阻值约为______Ω