Which of the following statements is true about PubMed?
A. PubMed houses 35 million citations and articles and all of them are free full-text versions
B. When searching it is required that Boolean operators are in all capitals
C. PubMed is good at increasing sensitivity of searches but not specificity
D. PubMed is a completely comprehensive database and if you use the right search, you will find every single article available for a certain topic