What is the 100th letter in the pattern ABCABCABC...?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the 100th letter in the pattern ABCABCABC...?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
A
What is the 100th letter in the pattern ABCABCABC...?
train