Which option is a natural material that is not a fossil fuel?

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Explanation:

1. Cotton (B)

2. It needs to go through a chemical reaction. (C)

3. A chain of repeating compounds found in both natural and synthetic materials. (D)

4. Oil and natural gas are chemically broken down into small compounds that combine to form plastic.

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.