Explanation:

- The given sentence is in the past tense, indicating that the action of being tired in the evening occurred before the present moment.
- The phrase "in the evening" indicates a specific point in time, suggesting that the action of being tired happened at that specific time.
- The phrase "because they" suggests that there is a cause and effect relationship between the action of being tired and the action that preceded it.
- The verb "helped" is in the past tense, indicating that the action of helping on the farm occurred before the present moment.
- However, the verb "helped" does not indicate the duration or continuity of the action. It only suggests that the action was completed in the past.
- To express the idea that they were tired in the evening because they had been helping on the farm continuously throughout the day, the past perfect continuous tense "had been helping" is used.

Example:
- They had been helping on the farm all day, so they were very tired in the evening.

Breakdown:
1. The sentence is in the past tense, indicating that the action of being tired in the evening occurred before the present moment.
2. The phrase "in the evening" indicates a specific point in time.
3. The phrase "because they" suggests a cause and effect relationship.
4. The verb "helped" is in the past tense, indicating that the action of helping on the farm occurred before the present moment.
5. However, the verb "helped" does not indicate the duration or continuity of the action.
6. To express the idea of continuous action throughout the day, the past perfect continuous tense "had been helping" is used.
7. The correct answer is "had been helping" to convey the continuous action of helping on the farm throughout the day.