Science is the systematic _________ of knowledge using nature itself or laboratory models and experiments.

A．the significance of this crisis
B．the inadequacy of governments
C．the similarity of the past to the present
D．the hopelessness of the situation

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