Q: Fact 1: Microorganisms can cause infections. Fact 2: Leeches clean away the dead or infected tissue. Given the two facts above, dead tissue can be caused by what?

Choose from:
a). disease;
b). pollen counts;
c). advanced mathematics;
d). microorganisms;
e). migrating birds;
f). viruses;
g). Physical weathering;
h). harmful substances;
The answer is:
d).