Which choice is the best way to determine how different levels of sand in soil affect the growth of cactus plants?
a Choose four different sand amounts, and place eight different types of cacti in the sands, two in each amount, and observe them every day.
b Choose one type of sandy soil and one type of cactus, and then observe the cactus for at least four weeks, measuring it each day.
c Choose five different sand amounts, and place one identical cactus in each sand amount for exactly one week.
d Choose three different sand amounts, and place four identical cacti in each amount to observe the sand’s effect on multiple plants.
