Twenty fruit flies are placed in each of four glass tubes. The tubes are sealed. Tubes I and II are partially covered with black paper; Tubes III and IV are not covered. The tubes are placed as shown. Then they are exposed to red light for five minutes. The number of flies in the uncovered part of each tube is shown in the drawing.
This experiment shows that flies respond to (respond means move to or away from):
A. red light but not gravity
B. gravity but not red light
C. both red light and gravity
D. neither red light nor gravity
because
A. most flies are in the upper end of Tube III but spread about evenly in Tube II.
B. most flies did not go to the bottom of Tubes I and III.
C. the flies need light to see and must fly against gravity.
D. the majority of flies are in the upper ends and in the lighted ends of the tubes.
E. some flies are in both ends of each tube.