Erica wants to know how many books on average are in school libraries in her county. she randomly calls 50 school libraries in her county and finds out how many books they have. she adds the number of books together and divides by 50. her conclusion is there are an average of 18,000 books in a school library. erica's conclusion is
a) invalid. her sample size is too small.
b) invalid. her sample size is too large.
c) valid. she used a representative sample.
d) bias. she wants schools to buy more new books.