Answer:
Step-by-step explanation:
I don't know what you have been told about this, but it seems to me there are more than 3 ways.
1. Many students studied for this test: they will get good grades.
2. Many students studied for this test; they will get good grades.
3. Many students studied for this test, and they will get good grades.
4. Many students studied for this test; moreover, they will get good grades.
#4 is the one you are likely missing. It uses a semicolon and a transitional expression (moreover).
#1 is kind of iffy. I use it because I like colons. They emphasize what is to follow, most commonly lists. It is likely not universally accepted. The other three are.