电阻R1的阻值为10Ω，将它与R2并联后，总电阻为5Ω，若将R1与R2串联，则总阻值为： [ ]
A.20Ω
B.15Ω
C.10Ω
D.5Ω
答案：A
I'm sorry I didn't phone you, but I've been very busy _____ the past couple of weeks. [ ]
A. beyond
B. with
C. among
D. over
主体倾斜观测，应在（）时间段内进行。
A.阴天.无风
B.强日照.无风
C.阴天.有较大风荷载
D.强日照.有较大风荷载
