Most Hot Springs That Are Just a Little Warmer Than

Most hot springs that are just a little warmer than mean annual temperature (6°-9°C) acquire their heat ________.

A) because most of the recharge from rainfall is in the warmer summer months
B) because most of the groundwater is millions of years old from when the Earth's climate was warmer
C) by deep circulation through progressively warmer rocks at depth (~2°C per 100 metres)
D) because there are young intrusions everywhere at depth, always warming up the crust