How Do We Know Whether or Not Global Warming Is

How do we know whether or not global warming is caused by humans?

A) From experiments in which a small experimental plot of forest is subjected to excess carbon, and the growth of the trees along with the temperature are then measured.
B) From computer calculations that "predict" 20th- century temperatures, based on natural variations, then on human- caused variations, and then on a mix of natural and human variations; the actual historical measurements then show the importance of human variations.
C) From experiments in which excess carbon is injected into a laboratory- created Earth- like atmosphere, and the resulting temperature is measured.
D) From computer calculations that "predict" 20th- century temperatures based on natural variations, then on human- caused variations, and then on a mix of natural and human variations; the actual historical measurements then show that only natural variations are important.
E) Actually, scientists have been unable to determine this.