What is the main ingredient added to fill the line when making a strawberry banana smoothie?

The primary ingredient added to fill the line in a strawberry banana smoothie is yogurt. Yogurt serves multiple purposes in a smoothie; it contributes creaminess, provides protein, and helps to blend the flavors together seamlessly. Additionally, yogurt enhances the texture of the smoothie, making it thicker and more satisfying.

In the context of smoothies, while milk can be used, it is typically considered more of a base liquid rather than a filling ingredient. Ice is mainly added to chill the smoothie and create a refreshing texture but does not provide the richness or nutritional benefits. Fruit puree is important for flavor but may not serve the same textural purpose as yogurt. This is why yogurt is the best choice for filling the line in this specific scenario, as it supports the overall quality and consistency of the beverage.