根据短文内容，选择正确答案。

    Tom walked into a shop. It had a sign outside: “Second-hand (旧的）clothes bought and sold.” He was carrying an old pair of trousers and asked the owner of the shop, “How much will you give me for these?” The man looked at them and then said,“Two dollars. ”

  “What?!” said Tom. “I thought they were worth at least five dollars.”

  “No,” said the man “They aren't worth a cent more than two dollars.”

  “Well,” said Toni, taking two dollars out of his pocket. “Here's your money. These trousers were hanging outside your shop. The list price (标价）of them was six dollars and a half. But I thought that was too much money, so I wanted to find out how much they were really worth.”

    Then he walked out of the shop with the pair of trousers and disappeared before the shop owner could think of anything to say.

 如图所示，R₁＝10Ω，R₂＝20Ω，R₃＝30Ω，电源电压恒定不变，若开关S₁闭合，S₂断开时，电流表的读数是0.3A。则电源电压为____ ；当S₁和S₂都断开时，电流表的读数为____ ；当S₁、S都闭合时，电流表的读数是____ 。