Line \(C\) is a straight line in the coordinate geometry plane defined by the equation \(y=ax+b,\) for which \(a\) and \(b\) are constants. What is the value of \(a?\)

(1) Line \(C\) is perpendicular to a line with the equation \(y=−2x+5.\)
(2) Line \(C\) is parallel to a line with the equation \(y=(1−a)x+b+2.\)

[spoiler]OA=D[/spoiler]

Source: Veritas Prep