The correct answer is: B. 20 to 30%
Silica is the main component of clay, and it is essential for making bricks. The percentage of silica in a good brick earth should be between 20 and 30%. If the percentage of silica is too low, the bricks will be weak and brittle. If the percentage of silica is too high, the bricks will be difficult to work with and may be prone to cracking.
Option A: 5 to 10%. This is too low a percentage of silica for a good brick earth. Bricks made from clay with this low a percentage of silica will be weak and brittle.
Option C: 50 to 60%. This is too high a percentage of silica for a good brick earth. Bricks made from clay with this high a percentage of silica will be difficult to work with and may be prone to cracking.
Option D: 70 to 80%. This is too high a percentage of silica for a good brick earth. Bricks made from clay with this high a percentage of silica will be difficult to work with and may be prone to cracking.