The vinegar removed only the shell of the egg because it was acidic. But it was unable to mess with the egg, because as it removed the shell of the egg, it was unable to get past the sac, the part that protects the inner egg. It is hard to puncture this sac, unless dropped or punctured by a nail or something hard.
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The correct answer is: Rinsing with alcohol during the washing step stripped the bacteria off the glass slide.
The sample may be lost during laboratory preparation. Usually the errors occur during the heating step (due to overheating or insufficient heating).
