【单选题】

If food is allowed to stand for some time, it putrefies .When the putrefied material　　is examined microscopically ,it is found to be teeming with bacteri

A、Where do these　　bacteria come from, since they are not seen in fresh foodEven until the mid-nineteenth　　century, many people believed that such microorganisms originated by spontaneous(5) generation ,a hypothetical process by which living organisms develop from nonliving matter.　　The most powerful opponent of the theory of spontaneous generation was the French chemist　　and microbiologist Louis Pasteur(1822-1895).Pasteur showed that structures　　present in air closely resemble the microorganisms seen in putrefying materials .He did(10)this by passing air through guncotton filters, the fibers of which stop solid particles.After　　the guncotton was dissolved in a mixture of alcohol and ether, the particles that it had　　trapped fell to the bottom of the liquid and were examined on a microscope slide .Pasteur　　found that in ordinary air these exists a variety of solid structures ranging in size from　　0.01 mm to more than 1.0 mm .Many of these bodies resembled the reproductive(15)structures of common molds, single-celled animals, and various other microbial cells.　　As many as 20 to 30 of them were found in fifteen liters of ordinary air ,and they could　　not be distinguished from the organisms found in much larger numbers in putrefying　　materials .Pasteur concluded that the organisms found in putrefying materials originated　　from the organized bodies present in the air .He postulated that these bodies are constantly(20)being deposited on all objects.　　Pasteur showed that if a nutrient solution was sealed in a glass flask and heated to　　boiling to destroy all the living organisms contaminating it, it never putrefied .The proponents of　　spontaneous generation declared that fresh air was necessary for　　spontaneous generation and that the air inside the sealed flask was affected in some way(25)by heating so that it would no longer support spontaneous generation. Pasteur constructed a　　swan-necked flask in which putrefying materials could he heated to boiling, but air　　could reenter. The bends in the neck prevented microorganisms from getting in the flask.. Material　　sterilized in such a flask did not putrefy. It can be inferred from paragraph 3 that Pasteur employed a swam-necked flask to
A、store sterilized liquids for use in future experiments
B．prevent heat from building up in a solution
C．disprove a criticism of his conclusions
D．estimate the number of organisms in a liter of air