For radiometric dating, you need to know which of the following? the sequence of rock layers a collection of fossil shells from sedimentary rocks the rate at which the radioactive decay occurs and how much has already occurred several types of radioactive material

Option C, the rate at which the radioactive decay occurs and how much has already occurred

Explanation:

The radiometric dating is based on studying the presence/existence of parents and daughter isotopes in a sample of any substance. Based on the abundance of both the parents and daughter isotopes, the rate of decay of any radioactive substance is determined.  The rate of decay of radioactive isotopes  is considered to be fixed. Therefore, once the rate of decay and "radioactive decay till date" is determined, the age of the substance can be easily calculated.

Thus option C is correct

"do a testcross with a homozygous recessive plant."