The development of the concept of isostasy (continental and oceanic crust being buoyed up by the denser mantle) resulted from indirect observations, as no one has been to the mantle to observe it holding the crust up. Which of the following observations was key to the development of this concept?

The slowing of the earth's rotation due to tidal friction

The decades-long deformation of the surveyor's line across the San Andreas fault 

Smaller deflection of the surveyor's plumb bob toward mountain ranges than expected 

Measurement of the amount of folding taking place in mountain belts such as the Alps and Himalayas Igneous rocks from the same location but forming at different times showing changes in their magnetic field orientation.