_____________ (应当注意的是) that pollution has become one of the thorniest problems in many big cities.

A．need graphic representations in order to understand higher-order language concepts
B．are good at studying English idioms but often fail to grasp higher-order language concepts
C．are not very patient with videodisc which helps them to understand the world concepts
D．tend to be deaf as well and have difficulty learning the simple concept "before and after"