Fill in the blank (_) in the following sentence from the following choices.
Sentence: Nelson stayed home from work and Kevin did not because _ had not built an immunity to the common cold.

Choice 1: Nelson stayed home from work and Kevin did not because Nelson had not built an immunity to the common cold.
Choice 2: Nelson stayed home from work and Kevin did not because Kevin had not built an immunity to the common cold.
Which of the choices is correct? Respond with only "1" or "2" and no additional text.
1
1.