This is my family．This is my father．He is strong．He has short hair．He is a doctor．This is my mother．She is not fat．She is a nurse．She has long hair．This is my grandfather．He is tall．He is a farmer．Where is my grandmother? She is in the kitchen．Look，that is my little brother．This is my big family．

小明利用热敏电阻设计了一个“过热自动报警电路”，如图甲。将热敏电阻R安装在需要探测温度高低的地方，当环境温度正常时，指示灯亮；当环境温度超过某一值时，警铃响。图甲中继电器的供电电压U₁=6V，继电器线圈的电阻R₀为30Ω，当线圈中的电流大于等于100mA时，警铃响。图乙是热敏电阻的阻值随温度变化的图象。下列说法正确的是（  ）