Question on energy transport in waves

A string under tension $T$ can support transverse waves with speed $c$. A displacement pattern of the form $y(x,t)=Acos(2πx/λ)cos(2πct/λ)$ is set up in the string. The energy transported in a time interval $Δt=λ/c$ across $x=0$ is..

The options are

A. 0

B. TA

C. Tλ

D. $frac{TA^2}{2λ}$

This is my work .. but doesn’t match I don’t know why?