_________ English can be difficult to learn, it is a very useful tool for communication.
During the scientific experiment, Mr. Chen carefully poured the water into the tube, with my classmates ____ beside to see what would happen.
My uncle gave me a ____ bag, in which there was a nice mobile phone as a present.
Rather than ____his car on the busy road, Mr. Green always prefers____ his bike.
-- Your telephone number again? I ____ quite catch it. -- It is 7657457.
A helicopter is able to ____ and land straight up and down.