How many variables should be tested in order for the experiment to valid?

A+b→2c when the reaction begins, the researcher records that the rate of reaction is such that 1 mole of a is consumed per minute. after making changes to the reaction, the researcher notes that 2 moles of a are consumed per minute. what change could the researcher have made to effect this change?

In a copper wire, a temperature increase is the result of which of the following

Fossil fuels are organic compounds that are made from