The Value $A ( t , r )$ Of an Investment of $4,000 After T Years in an Investment

The value $A ( t , r )$ of an investment of $4,000 after t years in an account for which the interest rate 100r% is compounded continuously is given by the function $A ( t , r ) = 4,000 e ^ { r t }$ dollars. Write the partial derivative $\frac { \partial A } { \partial t }$

A) $\frac { \partial A } { \partial t } = 4,000 r e ^ { n t }$
B) $\frac { \partial A } { \partial t } = 4,000 e ^ { rt }$
C) $\frac { \partial A } { \partial t } = 4,000 t e ^ {rt }$
D) $\frac { \partial A } { \partial t } = 4,000 r e ^ { r ( t - 1 ) }$
E) $\frac { \partial A } { \partial t } = 4,000 r e ^ { t }$