单项选择题
A.1~2 B.2~3 C.3~4 D.4~5
A.0.50 B.0.70 C.0.90 D.1.20
A.S1=707N S2=-500N S3=-500N B.S1=1414 N S2=500N S3=1000N C.S1=-707N S2=500N S3=500N D.S1=-707N S2=1000N S3=500N