For the following block diagram

what is the overall transfer function?

$($A$)\frac{\text{A}\text{B}\text{D}}{1+\text{A}\text{B}\text{C}-BD}$

$($B$)\frac{\text{A}\text{B}\text{D}}{1-\text{A}\text{B}\text{C}+BD}$

$($C$)\frac{\text{A}\text{B}\text{C}}{1+\text{A}\text{B}\text{D}-BC}$

$($D$)\frac{\text{A}\text{B}\text{C}}{1-\text{A}\text{B}\text{D}+BC}$