Answer:
A. the immune system could be conditioned.
Step-by-step explanation:
Ader and Cohen demonstrated that the immune system is also affected by psychological experiences and it could be conditioned according to them. They fed saccharin solution to mice and also injected them with Cytoxan at the same time. Cytoxan is an immuno-suppressing drug so it made the mice's immune system weak which made them sick.
After a long time of conditioning the mice, they fed them only saccharin solution. But the mice connected it with cytoxan injection so they stopped drinking it. Eventually they had to force feed the mice. Surprisingly, the mice started to show symptoms of immuno-suppression even to the point of fatality. Even though they were just given saccharin solution this time, their immune system had been conditioned to connect saccharin with cytoxan. As a result, even in absence of cytoxan, the immune system got suppressed. Hence, it was demonstrated that immune system could be conditioned.