A student conducts an investigation studying the effect of the amount of different types of fertilizer on plant growh rate. He takes 20 plants from 4 different plant types growing each one over the period of 60 days and one group gets no fertilizer, measuring and recording data every few days. He makes sure each plant gets the same amount of water, soil and sunlight. What is the biggest flaw in this experiment?

a No constants
b Different types of plants
c No control group
d Not a large enough sample size