Final answer:
The given equation is N1 + a12N2. Without additional information, it is not possible to determine whether dN1/dt is positive, negative, or zero.
Step-by-step explanation:
The given equation is N1 + a12N2. The question is about the rate of change of N1 with respect to time (dN1/dt). 
To determine the sign of dN1/dt, we need more information about the variables and their relationship. 
Without additional information, it is not possible to determine whether dN1/dt is positive, negative, or zero. 
Therefore, the correct answer is 4) There is not enough information to determine.