Sure! Let's go through each sentence and fill in the blanks with the appropriate form of the verb given in parentheses. I'll also explain why each verb form is correct.

1. Rickets **is** a children’s disease caused by a lack of vitamin D. (be)
- "Rickets" is a singular noun, so we use "is."

2. Measles **is** a dangerous disease for pregnant women. (be)
- "Measles" is also treated as a singular noun despite being plural in form, hence "is."

3. Statistics **is** a difficult course for people to understand. (be)
- In this case, "statistics" is often treated as a singular noun when referring to the field of study.

4. Statistics **show** that approximately 40 percent of all marriages in the United States **end** in divorce. (show / end)
- Here, "statistics" takes the plural verb "show" because it refers to multiple data points, and "marriages" also takes the plural verb "end."

5. The United States **consists** of 50 states. (consist)
- "The United States" (singular) requires the verb "consists" (third-person singular form).

6. Two hours **is** too long to wait, don’t you think? (be)
- Although "two hours" is plural, the phrase is regarded as a unit of time, thus we use "is."

7. Cattle **are** allowed to graze near the church. (allow)
- "Cattle" is plural, so we use "are." The verb is in passive voice.

8. The police **arrested** the thief recently. (arrest)
- "Police" is plural, so we use the plural past form "arrested."

9. Peaches and cream **are** delicious. (be)
- The phrase "peaches and cream" is treated as plural, hence the verb "are."

10. Peaches and cream **are** perishable and **should** be refrigerated. (be)
- Similarly, "peaches and cream" is plural, so we use "are."

11. The majority of the birds **migrate** for winter. (migrate)
- "The majority" is treated as a plural idea, so we use the plural verb "migrate."

12. The majority of the homework **is** useful. (be)
- "Homework" is a singular noun, thus "is."

13. A number of books on U.S. universities **are** in the library. (be)
- "A number of" indicates a plural quantity, so we use "are."

14. The number of books on U.S. universities **totals** fifteen. (total)
- "The number of" takes a singular verb, hence "totals."

15. Kara Walters, together with her classmates, **presents** a formidable opponent on the basketball court. (present)
- The subject "Kara Walters" is singular, so we use "presents."

Here are the completed sentences:

1. Rickets **is** a children’s disease caused by a lack of vitamin D.
2. Measles **is** a dangerous disease for pregnant women.
3. Statistics **is** a difficult course for people to understand.
4. Statistics **show** that approximately 40 percent of all marriages in the United States **end** in divorce.
5. The United States **consists** of 50 states.
6. Two hours **is** too long to wait, don’t you think?
7. Cattle **are** allowed to graze near the church.
8. The police **arrested** the thief recently.
9. Peaches and cream **are** delicious.
10. Peaches and cream **are** perishable and **should** be refrigerated.
11. The majority of the birds **migrate** for winter.
12. The majority of the homework **is** useful.
13. A number of books on U.S. universities **are** in the library.
14. The number of books on U.S. universities **totals** fifteen.
15. Kara Walters, together with her classmates, **presents** a formidable opponent on the basketball court.

Let me know if you need any further clarification!