Questions

Black  surfaces are good emitters of heat ( They are also good absorbers of heat). If the black surface is rough too, the emitting will be better. So (b) is the correct answer. Shining surfaces reflect back radiation. So if the black surface is also shining the effectiveness of emitting heat is lessened.

The size of gear is usually specified by pitch circle diameter.

International Day for Disaster Reduction, held every 13 October, celebrates how people and communities around the world are reducing their exposure to disasters.

The year 2017 focus is : Reducing the number of affected people by disasters by 2030.

The acid used in lead storage cells is Sulphuric acid. They are used car batteries. Lead-acid batteries are capable of being recharged, which is important for their use in cars.