Ukuleles(尤克里里)look like small guitars, but unlike guitars, they usually only have four strings. Ukuleles come from Hawaii(夏威夷)and make bright, happy sounds. They make people think of island vacations and summer days. Even though ukuleles come from Hawaii, they are now popular around the world. One of the reasons is because they are fun and easy to play.

    Playing a ukulele may be easy, but making one is not. Ukuleles must be made step by step. First the head and neck are glued to the body. As soon as the glue is dry, the fingerboard is glued onto the neck. Once(一旦)that is dry, the ukulele maker sands the fingerboard to make it smooth.

    Next, the bridge is glued onto the body. The bridge holds the strings and allows them to make sounds. After the bridge is in place, the ukulele can be painted and decorated. The next step is to connect tuning heads to the top of the ukulele. Then finally the strings are put on the instrument. Now a player can make music.

如图1所示，一个匝数n=100的圆形线圈，电阻r=1Ω．在线圈中存在面积S=0.3m²、垂直线圈平面（指向纸外）的匀强磁场区域，磁感应强度B随时间t变化的关系如图2所示．将其两端a、b与一个R=2Ω的电阻相连接，b端接地．试分析求解：

某兴趣小组以同一植物的枝条为材料，探究营养素和生长调节剂X对扦插枝条生根的影响，实验结果如图所示。下列推断不合理的是