The Thickness of the Crust of Mars Has Been Measured

Multiple Choice

The thickness of the crust of Mars has been measured to be different under two different hemispheres. How was this measurement made?

A) by radar measurements from an orbiting spacecraft, detecting radar waves reflecting back from the interface between crust and mantle on Mars
B) by drilling from Mars lander spacecraft into the crust of the planet, down to the mantle
C) by measuring the effect of slight differences in gravitational field upon the motion of a spacecraft orbiting Mars
D) by a seismology from the Mars landers, detecting the waves generated by Martian "earthquakes" as they passed through Mars