Which group of words in this sentence is a clause?

When World War II broke out, Bearden served in the U. S. Army.

Read the passage. (1) since prehistoric times, milk has been a vital source of nutrition for human beings. (2) some doctors and nutritionists believe that consuming milk can be detrimental to our health. (3) milk is one of the most nutrient-dense beverages we can consume and it should be an essential part of a healthy diet. (4) drinking just one 8-ounce glass of milk per day puts you well on your way to meeting recommended daily intakes for important nutrients. which transition would be best to use at the beginning of sentence 2? finally additionally however specifically