A is father of C and D is so...

All the organs of the body were having a meeting, trying to decide who was the one in charge.
"I should be in charge," said the brain, "Because I run all the body's systems, so without me nothing would happen."
"I should be in charge," said the blood, "Because I circulate oxygen all over so without me you'd all waste away."
"I should be in charge," said the stomach," Because I process food and give all of you energy."
"I should be in charge," said the legs, "because I carry the body wherever it needs to go."
"I should be in charge," said the eyes, "Because I allow the body to see where it goes."
"I should be in charge," said the rectum, "Because I'm responsible for waste removal."
All the other body parts laughed at the rectum and insulted him, so in a huff, he shut down tight.
Within a few days, the brain had a terrible headache, the stomach was bloated, the legs got wobbly, the eyes got watery, and the blood was toxic. They all decided that the rectum should be the boss.
The moral of the story? Even though the others do all the work... the @$$hole is usually in charge.