Question #N874

The first person to successfully orbit the Earth was Yuri Gagarin, a Soviet cosmonaut. Gagarin’s voyage, which was completed in 1961, was a momentous achievement in space exploration.

Which choice best combines the sentences at the underlined portion so that it conforms to the conventions of Standard English?
A. Gagarin’s voyage, completed in 1961, was a momentous achievement
B. Gagarin’s voyage, which was completed in 1961, was a momentous achievement
C. Gagarin’s voyage was completed in 1961, was a momentous achievement
D. Gagarin’s voyage, which was completed in 1961, was a momentous achievement