Which of the following is NOT a commercial grade hydrocarbon oil?

The reason B, No. 7 fuel-oil, is the correct answer is that this type of oil is generally not classified as a commercial grade hydrocarbon oil. The various grades of fuel oil are categorized based on their properties, such as viscosity and sulfur content, and No. 1, No. 2, and No. 5 represent common grades used in residential, commercial, and industrial heating applications.

No. 2 fuel-oil is widely used for residential heating and is a common choice for smaller burners. No. 1 fuel-oil is a lighter distillate that is used in specific applications requiring lower viscosity. No. 5 fuel-oil is typically a combination of heavier oils and is used in larger combustion equipment.

However, No. 7 fuel-oil is less common and not generally categorized within the mainstream commercial grades utilized for heating and industrial purposes, thus making it the appropriate choice for the question.