Consider the parametric equations x(t)=sint and y(t)=cos2t for 0<t<2π. Over which of the following intervals of t is the curve concave up with respect to x ?
a) 0<t<2π and 23π<t<2π
b) The curve is never concave up on 0<t<2π
c) 2π<t<23π
d) All t in 0<t<2π except t=2π,23π