Question: What is virus attenuation in practical terms for successful vaccine discovery?
A: Passage of live virus from cell to cell and a search for less virulent mutants
B: As (a) but using low temperature (33oC) to select cold adapted mutants
C: Direct random mutagenesis to create a less virulent mutant
D: Recovery of a naturally occurring 'wild' virus which is naturally less virulent
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option.