Which mathematical representation is correct for Fourier's law of conduction?

Which mathematical representation is correct for Fourier's law of conduction?

a. Q = k A (dx/dt)
b. Q = -k A (dx/dt)
c. Q = -k A (dt/dx)
d. None of the above

Correct Answer : c. Q = -k A (dt/dx)

Explanation :

Fourier's law states that the rate of flow of heat through a single homogeneous solid is directly proportional to the area of the section at right angle to the direction of heat flow, and change of temperature with respect to the length of the path of heat flow.