The correct answer is: C. Either I or II is implicit.
The PTA is a representative body of parents, and it is reasonable to assume that they would not take such a drastic step as to withdraw their children from the school unless they believed that a significant number of parents would support them. Therefore, assumption I is implicit.
However, it is also possible that the school authority may accede to the demand of the PTA even if only a minority of parents support the PTA’s position. For example, the school authority may be concerned about the negative publicity that they would receive if they were to ignore the PTA’s demands. Therefore, assumption II is also implicit.
In conclusion, both assumptions I and II are implicit in the statement.