[bjacokes]

See: non-response bias [1]

Parent comment should've said "could" instead of "will most likely", but their point is correct.

[1] https://en.wikipedia.org/wiki/Participation_bias

[mannykannot]

The article you link to establishes the reality of participation bias, but it does not exactly endorse option A. It does say "In e-mail surveys those who didn't answer can also systematically be phoned and a small number of survey questions can be asked. If their answers don't differ significantly from those who answered the survey, there might be no non-response bias. This technique is sometimes called non-response follow-up." This is not, however, the same as option A, which (as far as it goes) commingles responses from those who respond to the second prompt with those from the first, potentially concentrating a non-response bias in those who don't respond to either prompting. Furthermore, neither option A nor the above quote offers a remedy if evidence of non-response bias is found.