What is the energy source of this so-called solar-sail vehicle

A．Patent.
B．Invention.
C．Hope.
D．Prid

A．Most scientists are confident that the spacecraft will work well.
B．A more ambitious orbital flight will follow this mission.
C．The author is quite sure that this mission will make impossible technologies seem possible.
D．The key of the engineering of Cosmos I is building wings that can open and pivot.