关于电流，下面说法中正确的是（ ）
A．电子的定向运动形成了电流，电流的速度在大小上等于电子定向运动的速度。
B．在同样的电压下，金属导体中自由电子数越多电流就越大。
C．金属导体中电流越大，单位时间内流过导体横截面的自由电子数越多。
D．电流的单位是由电量和时间的单位导出的，这样的单位叫做导出单位。
答案：C
题目分析：电子的定向运动形成了电流，电流的速度等于光速，电子的速度远远小于光速，A错误；根据电流的围观表达式可得：
可得电流大，电子数目不一定多，B错误；根据公式
可得金属导体中电流越大，单位时间内流过导体横截面的自由电子数越多，C正确；电流的单位是基本单位，D错误
故选C
点评：要解答此题需掌握：物理学中把单位时间内通过导体某一横截面的电荷量叫做电流．在一定时间内，通过导体某一横截面的电荷越多，即电荷量越多，电流就越大．
阅读下面短文，从短文后各题所给的四个选项(A、B、C和D)中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。 Growing up for me was a challenge. I started out at primary school and was always asked by my teacher to sit at the 36 of the class because I was a fool. I kept 37 things: I forgot nursery songs and spellings. I was the 38 of class jokes and I was all alone and 39 . I hated school, but my dad would hear none of it. He kept telling me I was a 40 , if I believed it. A 41 around came one day when I remembered the 42 of the word “Cognoscenti”; a word all the other “ 43 students ” had forgotten how to spell 44 they spent their free time making fun of me. I wasn’t asked to spell, 45 I raised up my hand. So I stood up and went to the front of class, 46 23 pairs of eyes staring at me. My teacher grabbed her stick, ready to hit me if I’d 47_ . I wrote the letters, spelt the word and became an instant champion. Afterwards, I 48 and won, for my school, five Spelling Bee championships. Secondary school had its own share of challenges. I was a tall kid and wasn’t good at any 49 , except looking. I loved basketball and tennis. 50 I held a bat(球拍), I was laughed at by my opponent. He 51 six straight sets(盘，局) without sweat and there I was, sweating like I’d run a marathon, whereas I hadn’t even 52 a single point. My dad’s 53 kept playing in my ears “Stanley, you’re a champion if you believe it.” Believe it I did, because I wasn’t only good at tennis and basketball, I was an all-round athlete and 54 won both athletic and 55 scholarships to university.
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