The correct sequence for the perception of smell in mammals is?

Analyze the question: It asks for the correct and complete sequence of events in smelling (olfaction) in mammals.

Explain the concept: Smell works through these steps:

1. Odour molecules are carried in the air into the nasal cavity.
2. They dissolve in the moist mucus lining the nasal cavity. Only dissolved chemicals can stimulate the receptors.
3. The dissolved chemicals stimulate the olfactory (sensory) receptor cells in the olfactory epithelium.
4. These cells generate impulses that travel along the olfactory nerve.
5. The impulses reach the olfactory bulb and the brain, where the smell is interpreted.

Step-by-step evaluation of the options:

• A. It leaves out the dissolving step and treats "nerve endings" as the receptors. It is incomplete.
• B. It includes dissolved chemicals and sensory cells, but skips the olfactory nerve that carries impulses to the brain. It is incomplete.
• C. It does not say the chemicals must be dissolved, and it skips the nerve. It is incomplete.
• D. Dissolved chemicals → sensory cells → olfactory nerve → brain. It includes every key stage in the correct order.

Exam tip: For sense-organ sequences, remember Stimulus → Receptor → Nerve → Brain. For smell, add that the chemicals must be dissolved first. For taste, the same applies: chemicals dissolve in saliva. When several options look similar, choose the one that lists the most complete chain.