Question

3. Sam was absent and nobody informed him about the test. That‟s why he didn‟t study.
(because) 

Answer 1

because Sam was...didn't study

Answer 2

Sam was absent and no one informed him about the test because that's why he didn't study.

Answer 3

3. Sam was absent and nobody informed him about the test. That‟s why he didn‟t study.

-> Sam didn't study because he was absent and nobody informed him about the test.