Which of the following is NOT an ingredient in concrete?

Concrete is a composite material that typically consists of essential ingredients including water, gravel (or aggregate), and sand. Water is crucial as it activates the chemical reaction with cement, allowing the mixture to harden and gain strength. Gravel, acting as a coarse aggregate, provides the bulk and structural integrity of the finished concrete. Sand, being a fine aggregate, fills in the spaces between the gravel particles, enhancing the overall stability and density of the mix.

Wood, however, is not an ingredient in concrete. It belongs to a completely different category of building materials and does not contribute to the composition or properties of concrete. While wood can be used in conjunction with concrete in construction (such as formwork or framework), it does not chemically or physically integrate into the concrete mixture itself. Therefore, identifying wood as the non-ingredient amongst the choices accurately reflects its distinction from the fundamental components of concrete.