A Primary Reason for the Literary Digest Poll's Erroneous Forecast

A primary reason for the Literary Digest poll's erroneous forecast that Landon would win the 1936 presidential election was

A) the large proportion of undecided voters at the time of the poll.
B) the small sample upon which the poll was based.
C) an inadequate sampling frame that systematically excluded the poor.
D) the use of mailed questionnaires rather than more reliable face-to-face interviews.