Endocarditis is often a consequence of bacteria attached to the heart valves. These microbes are not eliminated by circulating lymphocytes because _______.

A) Gram-positive microbes don't have an exotoxin surface molecule to trigger an immune response
B) lymphocytes do not circulate through the heart valves to detect the presence of microbes
C) lymphocytes cannot engulf microbes attached to tissue
D) these microbes become covered in platelets and fibrin which is not recognized as foreign by the immune system