A magnetic compass needle oscillates 30 times per minute at a place where the dip is 45∘, and 40 times per minute where the dip is 30∘. If B1 and B2 are respectively the total magnetic field due to the earth at the two places, then the ratio B1/B2 is best given by:
a. 1.8
b. 0.7
c. 3.6
d. 2.2