21．The popularity of Chinese-owned video sharing service Tik Tok continues to grow among young people in the United States.As the Tik Tok app offers a wide choice of sounds，brief parts of songs and special effect tools to help create a video，it is especially popular with teens and young adults.
Tik Tok is considered a competitor to the video-sharing app Snapchat，as well as Facebook's Instagram service.Snapchat and Instagram also mainly interest young users with a wide mix of photos and videos centered largely on fashion，pop culture and humor.Last year，Tik Tok was the second—most downloaded app from Apple and Google stores.WhatsApp was downloaded the most.Research company Sensor Tower says people have downloaded the Tik Tok app 1.65 billion times.
Tik Tok has been used by many major companies to reach young people who use little or no traditional media.Companies appearing on Tik Tok can make money through advertising.Often，the companies create user "challenges" as part of their advertisement campaigns.In challenges，users are invited to publish their own videos in which they perform similar dance moves.
U.S.lawmakers have raised concerns about Tik Tok.They thought it is collecting and sharing users' data.Tik Tok has said it doesn't share information with others.Some groups and parents also have expressed concerns about videos.They thought the videos appearing on Tik Tok might be harmful to young users.A nonprofit privacy group notes on its website that many videos appearing on Tik Tok include
offensive language.So the group suggests that the service is best for those over age sixteen.
Tik Tok's head of creator partnerships，Kudzi Chikumbu，said the company is working hard to make sure the app is a "safe and positive environment".
（1）What is the most downloaded app on the Apple and Google stores？ （no more than 5 words）
（2）Why did some companies create user "challenges" on Tik Tok？ （no more than 10 words）
（3）As to Tik Tok，what did U.S.lawmakers worry about？ （no more than 15 words）
（4）What does the underlined word "offensive" in the fourth paragraph mean？ （1 word）
（5）What's your attitude towards the popularity of the video sharing service like TikTok？Please explain. （no more than 20 words）
