Question

now the pollution caused by the increasing number

now the pollution caused by the increasing number of cars _more and more serious in many cities.

Answer 1

now the pollution caused by the increasing number of cars _more and more serious in many cities.

填is
主语是the pollution
caused by the increasing number of cars是后置定语
more and more serious是形容词做表语
所以缺失系动词 is