Which of the following are thought to cause asthma?

The correct choice identifies both secondhand smoke and pollen in the air as factors that can contribute to the development or exacerbation of asthma.

Secondhand smoke is a known irritant and can trigger asthma symptoms or attacks in individuals who are sensitive to tobacco smoke. This exposure can inflame the airways and worsen respiratory issues over time.

Pollen, particularly during certain seasons when allergens are more prevalent, can also play a significant role in asthma exacerbations. Many individuals with asthma experience allergic reactions to pollen, which can provoke symptoms such as wheezing, shortness of breath, and chest tightening.

In contrast, good nutrition does not contribute to causing asthma; rather, it is important for overall health and may even help support respiratory health. Thus, the combination of secondhand smoke and pollen, which are established contributors to asthma, is accurately represented in the correct answer.