Knots in timber are A. Defects caused by crushing fibres B. Splits radiating from the centre C. Speckled strains D. Signs of branches cut off

The correct answer is D. Knots in timber are signs of branches cut off.

A knot is a protrusion on the surface of a piece of wood that is caused by the presence of a branch that has been cut off. Knots can be either tight or loose, depending on how well the branch was attached to the tree before it was cut off. Tight knots are more difficult to work with than loose knots, but they are also stronger.

Knots can affect the strength and appearance of wood, so they are often considered to be defects. However, knots can also add character to wood, and some people prefer the look of wood with knots.

Here is a brief explanation of each option:

• A. Defects caused by crushing fibres: This is not the correct answer because knots are not caused by crushing fibres. Knots are caused by the presence of a branch that has been cut off.
• B. Splits radiating from the centre: This is not the correct answer because knots are not splits. Knots are caused by the presence of a branch that has been cut off.
• C. Speckled strains: This is not the correct answer because knots are not speckled strains. Knots are caused by the presence of a branch that has been cut off.
• D. Signs of branches cut off: This is the correct answer because knots are caused by the presence of a branch that has been cut off.