Read the following statements carefully:
$Y:$ The resistivity of a semiconductor decreases with an increase in temperature.
$Z:$ In a conducting solid,the rate of collision between free electrons and ions increases with an increase in temperature.
Select the correct statement from the following:

• A
  $Y$ is true but $Z$ is false.
• B
  $Y$ is false but $Z$ is true.
• C
  Both $Y$ and $Z$ are true.
• D
  $Y$ is true and $Z$ is the correct reason for $Y$.