Read the passage.

(1) i think that schools should switch from using paper textbooks to using computer tablets. (2) textbooks were effective in the pre-digital age, but now we live in a technology-based society, so schools need to get with the program and adopt a modern approach to learning. (3) in fact, the chair of the federal communications commission said that "it’s time for the next stage” of learning with tablets and pointed out how textbooks are often out of date. (4) opponents argue that tablets aren’t a good choice because initially they’re very expensive. (5) the secretary of education pointed out that tablets can be updated regularly, which saves money in the long run. (6) not to mention the pluses of having the latest and greatest info! (7) many experts agree that switching to tablets is important for the future of education.
which phrase in sentence 2 should be revised to maintain a formal style?
a- "pre-digital age”
b- “technology-based society”
c- “get with the program”
d- “modern approach”