What is the figurative expression meaning if others may stumble ​

Read the passage. (1) the first skateboard was constructed of old roller-skate wheels attached to a piece of wood. (2) improvements were made to the skateboard in 1959. (3) the year that the first commercially made skateboard appeared. (4) soon after, skateboarding began to grow in popularity. (5) in 1963, skateboard manufacturers formed the first professional skateboarding team. (6) that same year, the first organized skateboard competition took place, in california. which is the most effective way to correct the fragment in sentence 3? attach the fragment to sentence 4. attach the fragment to sentence 2. insert the phrase in the united states to the end of the fragment. insert the verb had before the verb appeared.