18. For what nonzero values of k does the function y=Asinkt+Bcoskt satisfy the differential equation y′′+100y=0 for all values of A and B ? a. k=10 b. k=−100 c. k=−10 d. k=100 e. k=1 19. Which of the following functions are the constant solutions of the equation dtdy​=y4−y3+6y2 a. y(t)=2 b. y(t)=3 c. y(t)=5 d. y(t)=0 e. y(t)=et 20. Which of the following functions is a solution of the differential equation? y′′+16y′+64y=0 a. y=et b. y=te−8t c. y=6e−3t d. y=e−3t e. y=t2e−8t