Lex decides to examine the effect of fertilizer on the growth of tomato plants. Fie chooses four plants for his experiment and applies varying amounts of fertilizes to three of them. He does not apply fertilizer to one piant.
Over a 15-ay period, the plants receive fertilizer on Days 1, 4, 7, 10. and 13. Les measures the height of all of his plants with a metersick
on Days 3,6,9,12, and 15. He also makes sure to hold all experimental factors constant except for the fertilizer
What is the independent variable in Lexi's experiment on tomato plants?
the amount of fertilizer given to the plants
the days fertilizer was applied
the days plant height was measured
the measurements of plant height