Two Radio Antennas Are $130 \mathrm {~m}$ Apart on a North-South Line

Multiple Choice

Two radio antennas are $130 \mathrm {~m}$ apart on a north-south line. The two antennas radiate in phase at a frequency of $3.6 \mathrm { MHz }$ . All radio measurements are made far from the antennas. What is the smallest angle, reckoned east of north from midway between the antennas, at which constructive interference of two radio waves occurs? $\left( c = 3.00 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } \right)$

A) $55 ^ { \circ }$
B) $45 ^ { \circ }$
C) $35 ^ { \circ }$
D) $50 ^ { \circ }$
E) $40 ^ { \circ }$