Let x and y be functions of time t such that the sum of x and twice y is constant. Which of the following equations describes the relationship between the rate of change of x with respect to time and the rate of change of y with respect to time?
A.) dx/dt= 2 dy/dt
B.) dx/dt= -2 dy/dt
C.) 2dx/dt + dy/dt=0
D.) dx/dt + 2dy/dt = K, where K is a function of t