The Beaufort Scale is used to measure the intensity of tornados. For a tornado with Beaufort number B, the formula $v=1.9 B3/2 may be used to estimate the tornado's wind speed in miles per hour. Calculate the wind speed of a tornado with Beaufort number 9.
a) $39.9$ mph
b) $57.3$ mph
c) $69.8$ mph
d) $75.6$ mph