A Horizontally Projected Beam of Protons Travels At $55 \mathrm{~km} / \mathrm{s}$

A horizontally projected beam of protons travels at $55 \mathrm{~km} / \mathrm{s}$ into the space between two horizontal, parallel plates. The plates are separated by $7.4 \mathrm{~cm}$ , and they are $12 \mathrm{~cm}$ long. There is a uniform electric field between the plates of unknown strength. The beam enters the region exactly midway (vertically) between the plates and exits the region 7.2 microns closer to the lower plate. If it were a beam of electrons instead, everything else being the same, at what height above the lower plate would the beam exit the region between the plates? The proton mass is $1.67 \times 10-27 \mathrm{~kg}$ , and electron mass is $9.11 \times 10-31 \mathrm{~kg}$ .

A) $4.6 \mathrm{~cm}$
B) $2.3 \mathrm{~cm}$
C) $0.9 \mathrm{~cm}$
D) $5.0 \mathrm{~cm}$