Identifying Essential Ingredients in Baking

Oliver wanted to bake a cake for his friend's birthday, but he realized that he was out of flour. Without flour, he could not make the cake. Therefore, he decided to go to the store to buy some. Oliver knew that flour was a necessary ingredient for baking. He felt that it was important to have all the ingredients before starting. This situation left him with no choice but to take action quickly.

To imply the importance of flour in baking, we can say that flour is a(n) __________ ingredient.