A study of elementary school children, aged 6 to 11, finds a high positive correlation between shoe size x and score y on a test of reading comprehension. The observed correlation is most likely due to
a. the effect of a lurking variable, such as age.
b. a mistake, since the correlation must be negative.
c. cause and effect (larger shoe size causes higher reading comprehension).
d. "reverse" cause and effect (higher reading comprehension causes larger shoe size).