What's the No. 1 invention of the 20th century? The computer, say high school students surveyed for the "Invention Index" put out by the Lemelson-MIT Program. Their parents, however, picked antibiotics. Released this week, the "Invention Index" polled 300 students and 300 of their parents on the importance of various 20th-century inventions. In environment and energy, students chose recycling as the most important advance, while their parents picked solar power. But in the telecommunications and computing category, both groups overwhelmingly agreed the Internet was the most significant invention.