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C. A one (1) is assumed when there is no coefficient

Explanation:

The following chemical equation is given in the attachment:

2H2 + O2 → 2H2O

In this reaction, Hydrogen (H2) and oxygen (O2) gases are the reactants and they combine to form H2O, which is the product. It is necessary that, in accordance to the law of conservation of matter, the same number of atoms of each elements on the reactant side equates that on the product side. When this happens, the equation is said to be BALANCED.

However, to balance an equation, COEFFICIENTS are used. In this equation above, coefficients 2 is used in front of both H2 and H2O but no coefficient is used in front of O2. This is because 1 mole of oxygen is on the reactant and product side respectively. However, one is assumed when no coefficient is written i.e. O2 means 1 mole of oxygen molecule.

bcde is the incorrect answer it is a