Carbohydrates always contain:

• A. Nitrogen.
• B. Oxygen.
• C. Iron.
• D. Glucose.

Answer: (B)

Verified step by step guidance

1. Understand the composition of carbohydrates: Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O). They do not typically contain nitrogen, iron, or glucose as part of their basic structure.
2. Recall the general formula for carbohydrates: The basic formula for carbohydrates is \( C_x(H_2O)_y \), which indicates that they are made up of carbon, hydrogen, and oxygen atoms.
3. Eliminate incorrect options: Nitrogen is primarily found in proteins and nucleic acids, not carbohydrates. Iron is a mineral and not a component of carbohydrate molecules. Glucose is a type of carbohydrate, not an element or component.
4. Identify the correct answer: Oxygen is always present in carbohydrates, along with carbon and hydrogen, as part of their molecular structure.
5. Conclude the reasoning: Based on the molecular composition and the elimination of incorrect options, oxygen is the correct answer to the question.