Which of the following is true? A) glycine must be converted to serine before it is degraded B) serine must be converted

[answerhappygod]

Which of the following is true?
A) glycine must be converted to serine before it is degraded
B) serine must be converted to glycine before it is degraded
C) proline must be converted to aspartate before it is
degraded
D) glutamate must be converted to glutamine before it is
degraded
E) none of the above
The answer is A, but I want explanations why the others are
wrong and A is the answer