Given below are two statements: 

Assertion (A):	Houses made of concrete roofs overlaid with foam keep the room hotter during summer.
Reason (R):	The layer of foam insulation prohibits heat transfer, as it contains air pockets.

 
Choose the correct option from the given ones:

1.	(A) is True but (R) is False.
2.	(A) is False but (R) is True.
3.	Both (A) and (R) are True but (R) is not the correct explanation of (A).
4.	Both (A) and (R) are True and (R) is the correct explanation of (R).

A cup of coffee cools from \(90^{\circ}\text{C}\) $\mathrm{to}$ \(80^{\circ}\text{C}\) in \(t\) minutes, when the room temperature is \(20^{\circ}\text{C}.\)$\mathrm{}$ The time taken by a similar cup of coffee to cool from \(80^{\circ}\text{C}\) $\mathrm{to}$ \(60^{\circ}\text{C}\) at room temperature same at \(20^{\circ}\text{C}\) is:

Consider a compound slab consisting of two different materials having equal thicknesses and thermal conductivities K and 2K, respectively. The equivalent thermal conductivity of the slab will be:

We consider the radiation emitted by the human body. Which of the following statements is true:
 

A cylindrical rod has temperatures ${\mathrm{T}}_1$ $\mathrm{and}$ ${\mathrm{T}}_2$ at its ends. The rate of flow of heat is $Q_1$ cal/sec. If all the linear dimensions are doubled while keeping the temperature constant, then the rate of flow of heat $Q_2$ will be:

1. $4Q_1$

2. $2Q_1$

3. $\frac{Q_1}{4}$

4. $\frac{Q_1}{2}$

A black body has a wavelength \(\lambda_m\) corresponding to maximum energy at \(2000~\text{K}\). Its wavelength corresponding to maximum energy at \(3000~\text{K}\) will be:

Unit of Stefan's constant is:

1. Watt-m²-K⁴

2. Watt-m²/K⁴

3. Watt/m²–K

4. Watt/m² K⁴