The Precise Radius of the Inner Core Was Not Measured

Multiple Choice

The precise radius of the inner core was not measured until relatively recently.How was this accomplished?

A) by finding the S-wave shadow zone beyond 105°
B) by Neil Armstrong conducting hammer seismic experiments during a 1960's Apollo Mission
C) by using a precisely known, underground nuclear weapons test in the Nevada desert in the early 1960s
D) by measuring the edge of the P-wave shadow zone from the great Chilean earthquake of 1960