若一粒米的质量约是0.000021kg，将数据0.000021用科学记数法表示为（   ）

在“测定电源电动势和内阻”的实验中，针对两个不同的电源得出如右图所示的1、2两条图线，则两个电源的动势E₁和E₂、内阻r₁和r₂满足关系（    ）

阅读下面短文，从短文后所给的A、B、C三个选项中选出能填入相应空白处的最佳选项。

    The other day， I was getting into my car when I saw an old man walking up the hill. He looked tired， so I waited to see 1he was OK. He was fine， so I 2 him where he came from. He said that he just 3a train. He was almost blind， but I said he was4.

    We spent the next thirty minutes 5 about his life. He told me he invited 6 the army during the Second World War. And I even found 7 had a connection through a family member! We laughed so much that we almost cried.

    It cost me nothing other than thirty minutes of my 8. I learnt so much， and Albert enjoyed the talking and left smiling.

    I also had a(n) opportunity 9 to thank this great man for all he did to keep the country 10 during the war.

    Thanks， amazing Albert. You are an inspiration!