The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

Willpower is not some unchangeable characteristics we are born with. It is skill that can be developed, strengthened and targeted to help us achieve our goals. For every obstacle we want to overcome, we need willpower, which is an inner strength that will push us to confront challenge and keep us going.
Willpower can be（）