Why Do Scientists Think That the Current Global Warming Situation

Why do scientists think that the current global warming situation is NOT caused by natural forces?

A) Models of the Sun show that radiation hasn't varied throughout human history.
B) Volcanoes have not been known to cause global climate change, only local changes.
C) Mathematical models show that possible natural forces aren't significant enough.
D) Humans are the only sources capable of changing the climate.
E) They don't know. They just assume that humans are the cause.