Answer:
To be certain that the extract prepared from virulent cells still contained the transforming principle that was present prior to lysis, Avery incubated nonvirulent cells with the complete extract.
Step-by-step explanation:
When an extract prepared for incubation of microorganisms is stored for a very long period of time, this extract may lose its transformation principle. To test if the extract is still in good quality it is necessary that this extract is incubated with the microorganism for which it was made. If the microorganism develops, it means that the extract continues with the same transformation principle. Therefore, to be sure that the extract prepared from virulent cells still contained the transformation principle that was present before lysis, Avery should incubate non-virulent cells with the complete extract.