A Metallic Sheet Has a Large Number of Parallel Slits $5.0 \mathrm {~mm}$

A metallic sheet has a large number of parallel slits, $5.0 \mathrm {~mm}$ wide and $20 \mathrm {~cm}$ apart. This sheet is used as a diffraction grating for microwaves. A wide parallel beam of microwaves is incident normally on the sheet. What is the smallest microwave frequency for which only the central maximum will occur? $\left( c = 3.00 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } \right)$

A) $2.0 \mathrm { GHz }$
B) $1.0 \mathrm { GHz }$
C) $1.5 \mathrm { GHz }$
D) $0.70 \mathrm { GHz }$
E) $0.50 \mathrm { GHz }$