The ignition temperature of four substances p,q,r and s are 125 degree , 270 degree , 155 degree and 310 degree Celsius respectively .
Which of the following pairs of substances catches fire at 250 degree Celsius ?
A)q and s
B)p and r
C)r and s
D) p and q

Dear student, 
The temperature at which a substance starts burning or catches fire is called it's ignition temperature. For substance burn the temperature should be greater than or equal to ignition temperature. 
As p and r have ignition temperature 125°C  and 155°C which is less than 250°C.
Therefore p and r will catch fire. 
Option B is correct 

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