In Studying the National Concept of the Word 'Character' in Relation

In studying the national concept of the word 'character' in relation to morals and virtues,a researcher picks a simple random sample of 100 local flats and visits them to interview the occupants.After the first follow-up visit,the researcher has not been able to get in touch with the occupants in 19 of the properties.The researcher then chooses a second sample of 100 flats in a different area and continues until 100 responses have been received.What assumption affects any results which can be drawn from this study?

A) Human error
B) Overestimate of average results
C) Undercoverage bias
D) Reader-response bias