A biology student wants to determine if using a fertilizer would help promote growth of new babies in spider plants. He also believes that the type of spider plant may also affect the propagation. The student gets 90 plants, 30 of each spider plant variety: green, variegated, and curly. They all are potted in the same amount and type of soil, given the same amount of water, and exposed to the same amount of light. For each type of plant, the plants with similar sizes are placed in groups of two and labeled with the numbers 1 and 2. The letters A and B are written on equal-sized slips of paper and placed in a bag. The bag is shaken and, for the first group of two, a slip is drawn. If A is drawn, plant 1 will receive fertilizer and plant 2 will not receive fertilizer. If B is drawn, then plant 2 will receive fertilizer and plant 1 will not receive fertilizer. The slip is placed back into the bag and the bag is shaken. This procedure is repeated until all 90 plants have been assigned a treatment. After one year, the shoots will be counted for each plant. Which of the following describes the design for this experiment?

observational study
matched pairs design
randomized block design
completely randomized design