What does "brainchild" in Paragrapth 4 mean

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.

A．He remained in the saddle for several minutes spellbound.
B．He immediately jumped down and went forward.
C．He waited until his bed was ready and then dismounted.
D．He rode to the mound and stared at the structure before him.