Problem $15.$ Transfer Function for an Additive Process

Referring to the block diagram given below, which formula is the correct one for

$Y_3(s)?$

$($A$)Y_3(s)=G_1(s)+G_2(s)$

$($B$)Y_3(s)=G_1(s)G_2(s)$

$($C$)Y_3(s)=U(s)(G_1(s)+G_2(s))$

$($D$)Y_3(s)=\frac{G_1(s)}{G_2(s)}-\frac{Y_1(s)}{Y_2(s)}$