Bringing the nations of the world to act together in addressing environmental issues that spill over national borders requires$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$between countries with$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.$

a different approach to negotiations; similar income levels and sets of priorities

a different approach to negotiations; low and middle$-$income levels.

a difficult set of negotiations; different income levels and sets of priorities

deliberate negotiations; high$-$income levels and similar sets of priorities.