$A$ composite string is made up by joining two strings of different mass per unit length,$\mu$ and $4\mu$. The composite string is under the same tension $T$. $A$ transverse wave pulse,$Y = (6 \text{ mm}) \sin(5t + 40x)$,where $t$ is in seconds and $x$ is in meters,is sent along the lighter string towards the joint. The joint is at $x = 0$. The equation of the wave pulse reflected from the joint is:

• A
  $(2 \text{ mm}) \sin(5t - 40x)$
• B
  $(4 \text{ mm}) \sin(40x - 5t)$
• C
  $-(2 \text{ mm}) \sin(5t - 40x)$
• D
  $(2 \text{ mm}) \sin(5t - 10x)$