Solve the Problem $\mathrm { I } = 40 \sin 28 \pi \mathrm { t }$

Solve the problem.
-A generator produces an alternating current according to the equation $\mathrm { I } = 40 \sin 28 \pi \mathrm { t }$ , where $\mathrm { t }$ is time in seconds and $\mathrm { I }$ is the current in amperes. What is the smallest time $t$ such that $\mathrm { I } = 20$ ?

A) $\frac { 1 } { 112 } \mathrm { sec }$
B) $\frac { 1 } { 56 } \mathrm { sec }$
C) $\frac { 1 } { 84 } \mathrm { sec }$
D) $\frac { 1 } { 168 } \mathrm { sec }$