Automobile tunnels create huge ventilation problems. Even in the days of smoky, coal-burning engines, trains made their own ventilation by pushing and pulling air through the tunnels. Cars don't move their own exhaust in the same way; it collects in tunnels. Clifford M. Holland solved the problem in the 1920s with a giant fan-driven ventilation system for the Hudson River tunnel that bears his name.

Holland's system received a severe test in 1947 when a truck loaded with carbon disulphide caught fire in the tunnel and exploded. The blast destroyed twenty-three automobiles; sixty persons were overcome by the fumes. All sixty survived, however, and despite the ruin of 500 feet of the tunnel roof, traffic resumed after only fifty-six hours.

Q. Underline the words in the paragraph which in your judgment explain why the tunnel explosion was not fatal to many