$D(t)=10+4\mathrm{.}9cos(\frac{}{6}t)$

The function $D$ defined above models the depth, in feet, of the water $t$ hours after $12$ A$.$M$.$ in a certain harbor. Which of the following presents the method for finding the instantaneous rate of change of the depth of the water, in feet per hour, at $6$ A$.$M$.?$

$D^{\text{'}\text{'}}(6)=1\mathrm{.}343$

$D(6)=5\mathrm{.}100$