单项选择题
A.S<SUB>1</SUB>=707N S<SUB>2</SUB>=-500N S<SUB>3</SUB>=-500N B.S<SUB>1</SUB>=1414N S<SUB>2</SUB>=500N S<SUB>3</SUB>=1000N C.S<SUB>1</SUB>=-707N S<SUB>2</SUB>=500N S<SUB>3</SUB>=500N D.S<SUB>1</SUB>=-707N S<SUB>2</SUB>=1000N S<SUB>3</SUB>=500N