Read the paragraph: Electric buses can reduce exhaust fumes along busy routes. They are not, however, a simple plug-in replacement for every diesel bus. Cities must plan charging stations, train mechanics, and adjust schedules around battery range. The environmental benefit is real, but it depends on infrastructure that supports the vehicles. What does the sentence "They are not, however, a simple plug-in replacement for every diesel bus." mainly add to the paragraph's development of electric buses?