Consider two isolates, A and B. Microbe A had an LD₅₀ of 5 * 10⁴, while microbe B recorded an LD₅₀ of 5 *10⁷. Which isolate is more virulent?

A) unable to know based on information provided
B) microbe A
C) microbe B
D) They are equally virulent.
E) The values indicate that neither one is virulent.