شرکت در آزمون آنلاین انگلیسی نهم - انگلیسی نهم | آزمون شماره 3069

No sound can exist without movement. Sound is made by vibration and travels in waves. The waves move from the vibrating object in every direction, in ever-increasing circles. They disturb the molecules in the surrounding air, so that one molecule hits the next, and so on until the vibration reaches the ear. There, the eardrum vibrates, and the nerves transmit impulses to the brain, which gives them a meaning as sound. If the source of sound vibrates in a vacuum, no sound can be heard. This is because there is no surrounding material (i.e. molecules) to send the sound to the ears. The speed of sound is changed by the material used to send it. That is to say, it travels slowest in the air, faster in a liquid and fastest in a solid.

The vacuum has no surrounding material to let the sound …… .

Nowadays, children must … school regularly so that they can learn about different subjects such as chemistry, physics, and history.

Words like him, you, me etc. are called objective . . .  .