In the Last 20 Years Geologists Have Begun Measuring Accumulation

Multiple Choice

In the last 20 years geologists have begun measuring accumulation of Helium (He) in some minerals because these minerals can retain He at temperatures up 100-200°C.The "date" gives a time when the material cooled below the temperature where the helium began to be retained,which is related to the depth of the material.Where do you think the He comes from that allows this technique to work?

A) the atmosphere,like carbon-14
B) the sun
C) beta decay of some radioactive element
D) alpha decay of some radioactive element