Does B have a responsibility towards A?

1)A falls oss off B's roof while fixing B's roof. : B does not have respnibility towards A, because since A took the responsibility of fixing B's roof he should be more careful while walking on a broken roof.
2)B walks by A, A is homeless, asks for mon ey and B just walks on by. : Although it is not B's fault that A ended up homeless, i believe that it is B's moral obligation to help A and give A some money.
3)A needs to go somewhere, B gives him his car, breaks don't work and A has an accident. : It was B's responsibility that A had an accident. Since B gave his car to A then he should have it checked up first.
4) B has piggy virus, goes to school and gives it to A. : This is obviously B's fault because he shouldn't have gone to school when he had a contagous disease.