Attempts to restrict parking in the city center have further _________ the problem of traffic congestion, and sometimes the traffic comes to a complete stop in some places.

A．modified
B．ascertained
C．testified
D．violated

A．is a by-product of the oil-producing countries
B．is where to find new energy resources
C．can not be solved without coordinated efforts
D．is that no resources are inexhaustible