Air pollution and traffic congestion are two major ________ among city dwellers. A.concerns B.residents  C.unemployment D.infrastructure

The correct answer is A. concerns.

In this sentence, we are discussing the issues that affect city dwellers. "Air pollution" and "traffic congestion" are negative aspects of urban living that can cause worry or anxiety for the people living in cities. Thus, the word "concerns" fits perfectly because it denotes matters that require attention or are troubling.

The other options do not fit well in this context:
B. residents - This refers to the people living in the city, not the issues they face.
C. unemployment - While unemployment can be a significant issue, it is not directly related to air pollution and traffic congestion in this particular context.
D. infrastructure - This typically refers to the physical structures and facilities needed for the operation of a society, and while infrastructure can relate to traffic congestion, it does not encompass air pollution.

Overall, "concerns" is the most appropriate word that relates to both specified issues faced by city dwellers.