Wee Care Is a Nursery School for Pre-Kindergarten Children The Marginal Cost When the Twenty-First Student Enrolls in the School

Wee Care is a nursery school for pre-kindergarten children. The school has determined that the following biweekly revenues and costs occur at different levels of enrollment: $\begin{array}{rrr}\text { Number of }\\\text { Students Enrolled }&\text { Total Revenue } &\text { Total Costs }\\10 & \$ 3,000 & \$ 2,100 \\15 & 4,500 & 2,700 \\16 & 4,800 & 2,800 \\20 & 6,000 & 3,200 \\21 & 6,300 & 3,255\end{array}$ The marginal cost when the twenty-first student enrolls in the school is:

A) $55.
B) $155.
C) $300.
D) $3,045.
E) $3,255.