18. Which sentence in the paragraph is the topic sentence?

(1) There's no doubt that Springdale needs a new high school. (2) The old Washington High School on Main Street is in disrepair. (3) It will cost thousands of dollars to make it a safe and modern place for students and teachers. (4) Yet, even if all that money is spent, the building will remain cramped and overcrowded. (5) The better course is to invest our money in a new, modern building. (6) Several sites have been suggested. (7) For the sake of our young people—and our teachers—it's time to give this matter our serious attention.