If You Wanted to Test If Antibacterial Soap Worked Better

If you wanted to test if antibacterial soap worked better than regular soap, which experiment below has the strongest experimental design?

A) Have people wash with antibacterial soap first and later with regular soap.
B) Have two groups of people wash with either antibacterial or regular soap.
C) Have two groups of people wash with either antibacterial or regular soap, without knowing which type of soap they are using.
D) Have two groups of people wash with either antibacterial or regular soap, with neither the researchers nor the groups knowing which type of soap they are using.