单项选择题

请根据下面方块图写出传递函数式,正确的表达式是()。

A、G(S)=G1(S)X1(S)+G2(S)X2(S)+G3(S)X3(S)
B、G(S)=G1(S)X1(S)G2(S)X2(S)G3(S)X3(S)
C、G(S)=G1(S)+G2(S)+G3(S)
D、G(S)=G1(S)G2(S)G3(S)X3(S)