$A$ ladder leans against a wall such that its lower end makes an angle of measure $\theta$ with the ground. If its lower end is $a \ m$ away from the base of the wall,then the ladder reaches the wall at a height of $\ldots \ldots \ldots \ldots \ldots \ m$.

• A
  $a \cot \theta$
• B
  $a \tan \theta$
• C
  $a \sec \theta$
• D
  $a \operatorname{cosec} \theta$