[单选题]
4. What is the best nt2x-maiqo 0i.m. si;del6 4k j-5 pj:ticrx yc;rrl;1qscription of quorum sensing?
A. A response causing bioluminescence.
B. A response to population density that affects gene expression.
C. A response to danger to warn predators.
D. A hormonal response to high population density.
