The correct answer is - B. pull factor.
The intervening opportunity is considered as a significant pull factor for someone to migrate to a certain place. The pull factors are essentially things that are attracting someone to a certain place, thus migrate to that place because of the better living conditions in multiple aspects.
Intervening opportunity is the opportunity for settling at a certain place, less by distance or population pressure, at least at the starting point of the settling.
Option b. 100+
Some birds are nomadic in winter, others spend the colder months in the southwestern part of the continent or in the Mediterranean region. Geographical conditions determine several main routes. The Alps are an important barrier to migratory birds. About 150 travel westward and southwestward; others travel southeastward.
A problem caused by the burning of fossil fuels is acid rain. Hope this helped :D
