题干

如图所示,当人推墙时,人将向后退,对此现象的一些说法中正确的是(  )

A:人能后退一段距离是因为仍然受到墙的作用力

B:人能后退一段距离是因为人具有惯性

C:人停下来后惯性就消失了

D:人对墙的力与墙对人的力是一对平衡力

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答案(点此获取答案解析)

B

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    Did you know that badminton is also a fast-paced competitive sport? Badminton has been a Summer Olympic sport since 1992. Para-badminton will be included in the 2020 Summer Olympic Games in Tokyo. Here's what you need to know if your child is interested in badminton.

    Badminton is played on a 44-foot-long rectangular (矩形的) court that is 17 feet wide for singles and 20 feet wide for doubles. It is divided in the middle by a net placed 5 feet off the ground, like volleyball, not at ground level like tennis.

In competition, each match is a set of three games played to 21 points. There is a one-minute break when a player or team reaches 11 points, and a two-minute break in between the first and second games and the second and third games.

Age kids can start: About 6 years old.

    Best for kids who: Have good skills, are patient and hardworking, and can handle the pressure of one-on-one or two-on-two competition.

Team or individual: Matches can be singles (one-on-one) or doubles (two-on-two).

    Fun fact: Badminton is the only sport that has mixed doubles event in the Olympic Games.

    Costs: Beginners are expected to pay $30 to $40 for an entry-level racquet (球拍). Badminton shoes are similar in price to other athletic shoes. Join a club or team for lessons. A one-hour group lesson for beginners should cost $20 to $25, with costs increasing as your child moves up the ranks. You will also pay entry fees for competitions.

    Time commitment required: Beginners will play for just an hour or two per week.

    Potential for injury: Players have a very low risk of injury. Players who wear glasses should use eye protection.