A/an is the removal of the vitreous fluid of the posterior chamber of the eye and replacing it with a clear solution.

A Vitrectomy is the removal of the vitreous fluid of the posterior chamber of the eye and replacing it with a clear solution. This is often done in the posterior chamber because the vitreous humor sometimes get tangled with other major parts of the eye. 

b. 18

Step-by-step explanation:

From the information, we know that the clerk  put only packages of flour tortillas and packages of corn tortillas and that the ratio of the number of packages of corn tortillas to the total number of packages on the shelf was 7 to 16. This means that the rest of the packages which have a ratio of 9 to 16 are of flour tortillas. From this and the options given, you can infer that the number of packages of flour tortillas that the clerk could have put on the shelf is 18 because it is the only number that is a multiple of 9.