Gordon has written the following paragraphs about the Hubble Telescope: The Hubble Space Telescope (HST), which was named after the astronomer Edwin Hubble, orbits the earth and observes the far reaches of the universe in near-ultraviolet, visible, and near-infrared spectra. Its launch in 1990 was the fulfillment of almost 70 years of theory and planning—and almost 20 years of construction. When it was launched, the telescope contained five scientific instruments: a Wide Field and Planetary Camera (WF/PC), a Goddard High Resolution Spectrograph (GHRS), a High Speed Photometer (HSP), a Faint Object Camera (FOC), and a Faint Object Spectrograph (FOS). After many initial technological challenges, the HST was finally fully operational in 1993 and began capturing some of the most breathtaking pictures of the universe ever recorded. Which is a transitional phrase, and what is its purpose?

A. “the HST was finally fully operational”; to ensure that the text has a formal style

B. “When it was launched”; to provide a clear transition between the ideas in the two paragraphs

C. “the telescope contained five scientific instruments”; to add precise, relevant details to the second paragraph

D. “observes the far reaches of the universe in near-ultraviolet, visible, and near-infrared spectra”; to clarify meaning through the use of topic-specific vocabulary