题干

设m为不小于2的正整数,对任意n∈Z,若n=qm+r(其中q,r∈Z,且0≤r<m),则记fm(n)=r,如f2(3)=1,f3(8)=2,下列关于该映射fm:Z→Z的命题中,不正确的是(  )

A:若a,b∈Z,则fm(a+b)=fm(a)+fm(b)

B:若a,b,k∈Z,且fm(a)=fm(b),则fm(ka)=fm(kb)

C:若a,b,c,d∈Z,且fm(a)=fm(b),fm(c)=fm(d),则fm(a+c)=fm(b+d)

D:若a,b,c,d∈Z,且fm(a)=fm(b),fm(c)=fm(d),则fm(ac)=fm(bd)

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A

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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.