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: Anatomy
Limb buds appear at week: 
Select and explain. 
A. 6, B. 5, C. 3, D. 4
D: 4
Answer: D: 4 
Explanation: Upper limb buds appear by the end of week 4 and about 2 days later lower limb buds appear (beginning of week 5).