SAT Test 6: Sunflower growth - question #4-12

In 1919, H. S. Reed and R. H. Holland published a paper on the growth of sunflowers. Included in the paper were the table and graph above, which show the height h, in centimeters, of a sunflower t days after the sunflower begins to grow.
Over which of the following time periods is the average growth rate of the sunflower least?
A) Day 0 to Day 21
B) Day 21 to Day 42
C) Day 42 to Day 63
D) Day 63 to Day 84