Interpolation and extrapolation — ACT practice questions

In a shallow pond, snails sometimes climb plant stems until they are above the water. During an approaching storm, researchers measured air pressure, dissolved oxygen in the water, and the percentage of visible snails above water. At air pressures of 102, 100, 98, and 96 kPa, dissolved oxygen concentrations were 8, 7, 5, and 4 mg/L, respectively. The corresponding percentages of snails above water were 12%, 26%, 61%, and 78%.

Three students explained the pattern.

Student 1

Snails directly detect falling air pressure and climb in response. Dissolved oxygen happens to fall before storms but is not the cue. At low pressure, snails will climb even if dissolved oxygen remains high.

Student 2

Snails climb when dissolved oxygen is low because they can obtain oxygen from air. Air pressure does not directly affect them. At low dissolved oxygen, snails will climb even if air pressure remains high.

Student 3

Neither low pressure nor low dissolved oxygen is sufficient alone. Both changes must occur together before most snails climb.

Researchers tested the explanations using 4 identical chambers. Each chamber contained 20 snails in pond water at the same temperature. In Chamber A, pressure was 102 kPa and dissolved oxygen was 8 mg/L. In Chamber B, the values were 96 kPa and 8 mg/L. In Chamber C, they were 102 kPa and 4 mg/L. In Chamber D, they were 96 kPa and 4 mg/L. After 30 min, the percentages of snails above water in Chambers A through D were 14%, 69%, 72%, and 76%, respectively. Each chamber was tested 5 times with new groups of snails, and the reported percentages are means. Light level, water depth, and the number of plant stems were kept constant.