If \(x, y,\) and \(k\) are positive numbers such that \(\dfrac{x}{x+y}\cdot 10+\dfrac{y}{x+y}\cdot 20=k\) and if \(x < y,\) which of the following could be the value of \(k?\)

A. 10
B. 12
C. 15
D. 18
E. 30

[spoiler]OA=D[/spoiler]

Source: GMAT Prep