"if you teach that [slave] (speaking of myself) how to read, there would be no keeping him as it would forever unfit him to be a slave."
"He would a once become unmanageable, and of no value to his master."
These sentences are the best choices, because they both include key words to indicate that slavery and education are incompatible. In the first sentence, it states that teaching him to read makes him "unfit" as a slave. This shows that an education is incompatible to slavery. He then goes on to explain this when he says that the boy becomes "of no value to his master". This further shows it's incompatibility to slavery. The sentences about doing the boy harm do not work as support because they focus on the boy as a person rather than on him as a slave. The beginning also does not work for evidence because it is only explaining what Mrs. Auld taught him. The fact that slaves getting an education is illegal also doesn't show why they are incompatible - just that it's not allowed.
