5.1 a) To investigate the effect of gravity on the growth of the stem, one needs to compare a setup where gravity's directional effect is present with one where it is nullified. • In Diagram B, the plant is horizontal and stationary, allowing gravity to continuously act on the lower side of the stem, leading to a gravitropic response. • In Diagram A, the plant is horizontal but rotating on a clinostat. This rotation ensures that the gravitational stimulus is evenly distributed around the stem, effectively cancelling out any directional gravitropic response. By comparing the growth in Diagram A and Diagram B, the effect of gravity can be determined. The two diagrams are $\boxed{\text{A and B}}$. b) To investigate the effect of auxins on the growth of the stem, one needs to compare a setup where auxins are present and functional with one where they are absent or their production is inhibited. • In Diagram B, the plant is horizontal and stationary with its tip intact. The tip is the primary site of auxin production, so auxins are present and can mediate a gravitropic response. • In Diagram C, the plant is also horizontal and stationary, but its tip has been cut. Removing the tip eliminates the primary source of auxins. By comparing the growth in Diagram B (auxins present) and Diagram C (auxins largely absent), the role of auxins in stem growth can be investigated. The two diagrams are $\boxed{\text{B and C}}$. What's next?