在绝缘的水平桌面上有MN、PQ两根平行的光滑金属导轨，导轨间的距离为l．金属棒ab和cd垂直放在导轨上，两棒正中间用一根长l的绝缘细线相连，棒ab右侧有一直角三角形匀强磁场区域，磁场方向竖直向下，三角形的两条直角边长均为l，整个装置的俯视图如图所示，从图示位置在棒ab上加水平拉力，使金属棒ab和cd向右匀速穿过磁场区，则金属棒ab中感应电流i和绝缘细线上的张力大小F随时间t变化的图象可能正确的是（规定金属棒ab中电流方向由a到b为正）（   ）

下列各项史料中，能够反映南宋都城临安面貌的是（    ）

    Are you looking for some new and exciting places to take your kids to? Try some of these  places:

    Head to a natural history museum. This is where kids can discover the past from Dinosaur (恐龙) models to rock collections and pictures of stars in the sky. Also, ask what kind of workshops and educational programs are prepared for kids and any special events that are coming up.

    Go to a Youtheater. Look for one in your area offering plays for child and family visitors. Pre-show play shops are conducted by area artists and educators where kids can discover the secret about performing arts. Puppet (木偶) making and stage make-up are just a couple of the special offerings you might find.

    Visit art museums. They offer all kinds of activities to excite your kids' interest. Many offer workshops for making hand-made pieces, traveling exhibits, book signings( 签名) by children's favorite writers, and even musical performances and other arts.

    Try hands-on science. Visit one of the many hands-on science museums around the country. These science play-lands are great fun for kids and grown-ups alike. They'll keep your child mentally and physically active the whole day through while pushing buttons, experimenting, and building. When everyone is tired, enjoy a fun family science show, commonly found in these museums.

已知cosα=-$\frac{4}{5}$，sinα=$\frac{3}{5}$那么角2α的终边所在的象限为（　　）

回答下列问题

（1）氧气可压缩储存于钢瓶中，因为受压时氧气分子间的____减小；

（2）CH₄充分燃烧可用化学方程式表示为　____　；

（3）炒菜时锅里的油不慎着火，可用锅盖盖灭，其灭火原理是____．