It Is Known That Birds Can Detect the Earth's Magnetic $1.2 \mathrm {~m}$

Multiple Choice

It is known that birds can detect the earth's magnetic field, but the mechanism of how they do this is not known. It has been suggested that perhaps they detect a motional emf as they fly north to south, but it turns out that the induced voltages are small compared to the voltages normally encountered in cells, so this is probably not the mechanism involved. To check this out, calculate the induced voltage across the wingtips of a wild goose with a wingspan of $1.2 \mathrm {~m}$ if it is flying directly south at $13 \mathrm {~m} / \mathrm { s }$ at a point where the earth's magnetic field is 5.0 ×10^-5 T directed downward from the horizontal by $40 ^ { \circ }$

A) 0.50 mV
B) 0.60 mV
C) 0.78 mV
D) 0.060 mV
E) 0.25 mV