This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which of the following is a polar amino acid? 
Select and explain. 
A. Tryptophan, B. Serine, C. Valine, D. Tyrosine
B: Serine
Answer: B: Serine 
Explanation: Note: Polar amino acids are hydrophilic and nonpolar amino acids are hydrophobic.