The writer wants to combine sentences 18 and 19 (reproduced below) to emphasize the relationship between the ideas in these sentences.

It proved an invaluable tool for research projects involving secondary sources. It was more efficient to read scholarly articles on my e-reader than to go to the library for hard copies.

Which version of the underlined text best accomplishes this goal?

A. sources, while it

B. sources because it

C. sources, yet it

D. sources, although it

E. sources, unless it