题目内容

将一个恒定的直流电压通过电力电子器件的开关作用直接变为另一固定的或可调的直流电压的过程，称为直流-直流变换，也称为（）技术。

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升压斩波电路电压升高原因：（）储能具有电压泵升的作用；（）可将输出电压保持住。

以下（）不满足触发电路的要求。

A. 触发电路输出的脉冲必须具有足够的功率
B. 触发脉冲必须与晶闸管的主电压保持同步
C. 触发脉冲能满足主电路移相范围的要求
D. 触发脉冲必须是窄脉冲

单结晶体管自激震荡电路输出的电压波形是（）。

A. 锯齿波
B. 正弦波
C. 矩形波
D. 尖脉冲

Watch the video clip and fill in the blanks.Pay attention to the present continuous and the prepositions.1. The Clarks (1)such a good time! It's a pleasant evening. Paul and his parents (2)a comedy. It's so funny. They(3)a lot.2. Lucy and her mom (4) the sofa. Lucy(5) her doll. Oliver(6)his toy car on the floor.3. Lucy and Oliver(7)the comedy.4. Olivia is in her room. She(8) her desk. She ___(9)_ her laptop.5. Mrs. Clark(10) now.6. "Where are you going, honey?""I'm going to the kitchen. I have to make dinner.""I'm (11) ,. I want to help you."7. Mr. and Mrs. Clark (12) dinner now. They like to cook together. Mrs. Clark (13)the vegetables and her husband _(14)_ the food.8. Dinner is ready now, but Olivia is missing.9. "Where is Olivia? Paul, please go to her room and tell her we(15) her."10. "What are you doing, Olivia?""I'm (16) my friend, Amy.""Dinner is ready. Everyone (17) you.""I'm (18) it now. I'm coming!"11. The Clarks (19) dinner now. They (20) it.12. It's bedtime. Paul is going to the bathroom. He _(21)his teeth.13. "Mom, what are Lucy and Oliver doing? Are they (22) ?"No, they (23) . They (24)a bath.14. "And they (25) water all around! They are having so much fun!"15. Oliver and Lucy are in bed now. Their mom (26) them a bedtime story.16. Olivia (27) her bed and she (28)some photos on her phone.17. Her parents (29) the door They (30) the door.18. "Olivia, are you (31)?""No, I'm not. I'm just looking at some photos."19. Her mom (32)‍ her phone. "It's very late. It's time to go to sleep, my dear."20. Olivia's parents (33). Olivia is sleeping now.21. Paul can't sleep. He is going on a class trip tomorrow. He is (34) it. He is very (35).