when encountering merge activities during resource levelling , which assumption has the most bearing?

Question

1. Once started an activity may not be interrupted (assumption 1)
2. Level of resources used for an activity cannot be changed. (Assumption 2)
3.  An activity must be finished before any of its successors can be started. (assumption 3)
4.  The basic resource scheduling problem features only one type of resource that is available for each period with its full capacity. this type of resources is referred to as renewal. assumption 4