Daisy made the graph shown to display the data recorded during an experiment. Daisy is studying the effect of soil on the growth of plants. Three identical plants having a height of 15 cm were used for the experiment. The first plant was planted in Type A soil in 4 hours of sunlight, the second plant was planted in Type B soil in 6 hours of sunlight, and the third plant was planted in Type C soil in darkness. All other growing conditions of the plants were kept constant. A bar graph titled Plant Growth is shown. The y axis, labeled Height in centimeters, goes from 0 to 3.50. The x axis, labeled Week, goes from 1 to 4. Each week has 3 bars, Type A soil, Type B soil, and Type C soil. For weeks 1 to 4, bars for Type A soil have heights of 1.25 centimeter, 1.50 cm, 1.75 cm, 2.00 cm, bars for Type B soil have heights of 2.00, 2.25, 2.50, and 2.75 centimeters, and bars for Type C soil have heights of 0.50, 0.75, 1.00, and 1.75 centimeters.

What is wrong with Daisy's experimental design?

A single variable was used
Sunlight was not kept constant
More than one type of soil was used
Other growing conditions were not changed