To find their average age at present, follow these steps:

1. Determine Their Ages 5 Years Ago:
The average age of the brother and sister 5 years ago was 35 years. Since there are two of them:
$$\text{Total age 5 years ago}=2\times 35=70\text{ years}$$

2. Determine Their Current Ages:
Since 5 years have passed, each person's age has increased by 5 years. Therefore, their combined age has increased by:
$$2\times 5=10\text{ years}$$

So, their current total age is:
$$70+10=80\text{ years}$$

3. Calculate the Current Average Age:
$$\text{Average age at present}=\frac{80}{2}=40\text{ years}$$


The average age of the brother and sister at present is 40 years.

The correct answer is B. 40 years.