A body cools according to Newton's law of cooling from $100^\circ C$ to $60^\circ C$ in $20$ minutes. The temperature of the surroundings being $20^\circ C$, then the total time required for the body to cool down to $30^\circ C$ is
- A$90$ minutes
- B$1$ hour and $10$ minutes
- C$80$ minutes
- D$60$ minutes
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