At t =-2 ms, a sinusoidal voltage is known to be zero and going positive. The voltage is next zero at t = 8 ms. It is also known that the voltage is 80.9 V at t = 0. What is the frequency of v in hertz? Select the correct expression for v. v = 138 cos(100 pi t - 3 pi/10)V v = 69 sin(100 pi t - 3 pi/10) V v = 138 sin(100 pi t + 3 pi/10) V v = 69 cos(100 pi t - 3 pi/10) V v = 138 sin(100 pi t - 3 pi/10) V v = 138 cos(100 pi t + 3 pi/10) V