Identify the true statement.

a. Strip mining can be done economically down to a depth of about 1 km.

b. Anthracite coal develops at depth along mountain belt borders, where temperatures reach 600°C.

c. Underground coal mining is dangerous because of possible tunnel collapse and possible methane gas explosions.

d. Coal began to form on the Earth soon afterlife evolved in the Precambrian.

Option (C)

Explanation:

Coal is a sedimentary rock that forms due to the accumulation of dead matters of plants, trees, and converting it into coal over millions of years.

This coal is extracted from the earth in two ways-

Opencast mining - Here, in order to extract the coal, large strips of land are being excavated. Here, the construction of the tunnel is not required. It is comparatively safe and affects the environment in both long as well as short term. Underground mining - Here, the excavations and sub-surface tunnels are made in order to extract the required materials from a certain depth. This method is risky because the underground tunnels and the roof rocks can get the collapse and there may occur the methane gas explosion, and causes a fire, which is why it is not very safe to carry out underground mining.

Thus, the correct answer is option (C) - which is true.