The passage mentions that older people are more likely to lose their intelligence due to breathing unsafe air. The key factor here is age, particularly above 64 years, which is the age group most vulnerable to cognitive decline due to pollution.
- Option (a) talks about people below 64 years of age, but the passage does not focus on this age group.
- Option (b) suggests people of all ages, but the passage specifically mentions older individuals.
- Option (d) refers to the loss of the ability to think and speak, which is not mentioned in the passage as a direct consequence of unsafe air.
Thus, the correct answer is (c).