1.Air is not the only medium through which sound can travel. It also travels through water. In fact, sound travels 4,800 feet per second through water, and only 1,100 feet per second through air. The speed of sound through water is an important navigational aid to submarines. By emitting a series of sound pulses and measuring how long it takes for them to bounce back, submariners are able to tell how close they are to other objects. If it takes two seconds for a sound to travel through water to an object, how far is it from the submarine to the object? Yes, it's _______.

① 1,100 feet  

② 2,200 feet 

③ 2,400 feet

④ 4,800 feet

⑤ 9,600 feet

2.Have you ever heard about bookworms? Well, a bookworm may be a person who likes to read books, but it's also the name given to a bug that lakes to eat books. Many insects ruin books by eating the glue, paper and bindings that hold the books together One insect that does a lot of damage to books is the silverfish. Silverfish are covered with silver scales and move quickly. They are found in libraries as often as human bookworms are. However, human bookworms are there to read books, while silverfish are there to _______.

① eat books

② take a rest

③ stick books together

④ make people enjoy themselves

⑤ cover themselves with silver scales