Which of the following could base 2 bind with?
1. a guanine
2. a pyrimidine
3. an adenine
4. a purine

Base 2 is a purine so it can't bind to itself, so 1. and 4. are already out. 
You should review Chargraff's rules. 

The basic repeating unit of nucleic acids are known as nucleotides. a nucleotide consists of three distinct chemical groups.

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