In this problem, \( a, b, c \), and \( d \) are nonzero integers. If \( \frac{a}{b} \) is added to \( x \), the sum is \( \frac{c}{d} \). Which statement can be used to prove that \( x \) must be a rational number?
\( x=\frac{c-a}{d-b} \)
\( x=\frac{c b+a d}{b d} \)
\( x=\frac{c+a}{d-b} \)
\[
x=\frac{c b-a d}{b d}
\]