For the function given below, find a formula for the Riemann sum obtained by dividing the interval $2,6$ into $n$ equal subintervals and using the right$-$hand endpoint for each $c_k.$ Then take a limit of this sum as $n$ to calculate the area under the curve over $2,6.$

$f(x)=3x$

Which of the following expressions gives the formula for the Riemann sum for the given function over the interval $2,6?$

A$.{\displaystyle \underset{k=1}{\overset{n}{}}}3(2\frac{4k}{n})(\frac{1}{n})$

B$.{\displaystyle \underset{k=1}{\overset{n}{}}}3(2\frac{4k}{n})(\frac{4}{n})$

C$.{\displaystyle \underset{k=1}{\overset{n}{}}}3(2\frac{k}{n})(\frac{4}{n})$ D$.{\displaystyle \underset{k=1}{\overset{n}{}}}3(2\frac{k}{n})(\frac{1}{n})$

Simplify the sigma notation to find the sum in terms of $n$ only. Choose the correct answer below.

A$.S_n=\frac{24}{n}$ B$.S_n=48\frac{n}{24}$

C$.S_n=\frac{n}{24}$ D$.S_n=48\frac{24}{n}$

The area under the curve over $2,6$ is $$ square units.

$($Simplify your answer.$)$

Please help with the square units over the area of $2,6$