Which of the following pairs gives a correct combination of the useful and harmful constituents respectively of a good brick earth? A. Lime stone and alumina B. Silica and alkalies C. Alumina and iron D. Alkalies and magnesium

The correct answer is B. Silica and alkalies.

Silica is the main component of brick earth, and it is responsible for the strength of the brick. Alkalies are harmful to brick earth because they can cause the brick to break down over time.

Lime stone is not a component of brick earth, and it is not harmful to brick earth. Alumina is a component of brick earth, but it is not as important as silica. Iron is not a component of brick earth, and it is not harmful to brick earth. Magnesium is not a component of brick earth, and it is not harmful to brick earth.

Here is a more detailed explanation of each option:

• Option A: Lime stone and alumina. Lime stone is not a component of brick earth, and alumina is not as important as silica. Therefore, this option is not correct.
• Option B: Silica and alkalies. Silica is the main component of brick earth, and it is responsible for the strength of the brick. Alkalies are harmful to brick earth because they can cause the brick to break down over time. Therefore, this option is correct.
• Option C: Alumina and iron. Alumina is a component of brick earth, but it is not as important as silica. Iron is not a component of brick earth. Therefore, this option is not correct.
• Option D: Alkalies and magnesium. Alkalies are harmful to brick earth, and magnesium is not a component of brick earth. Therefore, this option is not correct.