长方形的四个角都是____角。

如图甲所示，是一种电加热恒温箱的简化工作原理电路图。工作电路由电压U₀ = 220V的电源和阻值为R₀ = 88Ω的电热丝组成。控制电路是由电压U₁ = 7.5V的电源、开关、电磁继电器（线圈电阻不计）、电阻箱R₁（可取值范围为 0～120Ω）和热敏电阻R_t组成的，热敏电阻R_t的阻值随温度变化的关系如图乙所示。当控制电路的电流达到50mA 时，衔铁才吸合，从而切断右边工作电路，停止加热。

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根据短文内容，选择正确答案。
D
    John sent for a doctor because his wife was ill. He called the doctor and made sure they would meet at five. He arrived at the doctor's at twenty to five. He thought. “It s a little bit earlier. I'll wait for a moment. It's good to keep the time. ”
    Then he stopped his car in front of the doctor's. He looked around and saw a noisy square not far from here. He went there and sat down on a chair to enjoy the last light of afternoon sun and make himself quiet. He saw some children playing and some women talking to each other happily.
    Suddenly he heard a girl crying. He came up to her and asked some questions. Then he knew she got lost. John tried to find out her address and took her home. The girl's parents were very thankful.
    Then John hurried to the doctor's. The doctor said angrily when he saw him “You're late. Why did you keep me waiting for 20 minutes?" John said nothing but one word "Sorry!”

某同学用图所示装置做探究杠杆平衡条件的实验，图中杠杆匀质且均匀刻度．当杠杆水平平衡后，在左侧第2格挂3个相同的钩码，则应在右侧第3格上挂____个相同的钩码才能使杠杆水平平衡；若在右侧改用弹簧测力计向下拉，弹簧测力计由竖直方向逐渐向左转动，杠杆始终保持水平平衡，则弹簧测力计的示数将逐渐____（选填“增大”、“减小”或“不变”）．