地球上产生昼夜交替现象的根本原因是(        )

$\frac{3}{6}$  =  ____  $\frac{7}{21}$  = ____        $\frac{18}{12}$  = ____       $\frac{27}{54}$  = ____

如图所示电路，开关闭合时观察到：L₁和L₂两灯均不亮，电流表无示数，电压表有示数，其原因可能是

                                                                                               House of Porridge
    Welcome to Clark's Porridge House. We have different kinds of porridge. We have meat porridge, vegetable porridge. And fruit porridge is our special, it is only RMB 4 yuan for one bowl. A bowl of meat is RMB 5 yuan. Vegetable porridge is RMB 3 yuan for one bowl, but RMB 5 yuan for two. We have thin pancakes, too. They are RMB 5 yuan for ten. Come and taste the delicious food here. If you are too busy to come, make a telephone call to us and we can send the food to you in 20 minutes.
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