If Your Immune System Has Memory Cells for Every Cold

If your immune system has memory cells for every cold virus you've ever had, why can you still catch colds?

A) Memory cells don't last very long.
B) Memory cells aren't produced in response to a viral infection.
C) Cold viruses change so quickly that memory cells from previous colds won't recognize the new versions.
D) Viral infections suppress one's immune response.
E) Memory cells only work on bacteria.