Which one is gramatically correct to say? A)a three-month's period
B)a three-month period
C)'A' and 'B' are both correct.

“A three-month period”

Explanation:

A could be used depending on what style of writing you’re doing and how you’re taught, so C could be an option. But your teacher would most likely mark it as wrong. B is the most widely-used term, so that’s correct.

the correct answer would be:  

a. adverb phrase

either online or traditional classroom can work for students. which is better will depend on students and teacher. students may have trouble following a lesson in classroom while they can study at their own pace online. other students may have trouble staying focus on their own in online classes. teachers are more used to classroom teaching but they may also be more worry about their jobs teaching online classes and work harder.