放在水平地面上重500N的木箱,受到水平向右100N推力时,恰好做匀速直线运动,则木箱受到的摩擦力是____ N.
①三大改造结束②抗美援朝③家庭联产承包责任制实施④土地改革开始⑤国企改革开始
The World Health Organization has given some advice on how to protect us from infected(感染) with H7N9.
生物分类单位由大到小依次是( )