It is related to tenses.

In this case simple present tense will be preferred over the 'ing' form (still continuing vs continues). C option says that Polio is going on while we are reading --- it should be simple present actually.

Best,
Gaurav

C

B is incorrect. If you read this sentence after deleting the words betwen the two commas, then it should make proper sense. Here it reads "Polio it still continues elsewhere and can be". Polio it is incorrect.

Also, the word "still" is redundant.

Hope this makes sense.

C is not correct. 1. It starts with "of Americans born before 1905", which makes the use of " of them " abosultely wrong 2. "have suffered" is wrong since this happened in past 3. "by age seventy-five" is not correct as well Option A takes care of these proble...