Read the following paragraph and underline the topic sentence. 　The very act of stepping outside of your comfort zone is critical to your success. Our brains are wired so that it’s difficult to take action until we feel at least some stress and discomfort. In fact, performance peaks when we’re well out of our comfort zone. If you’re too comfortable, your performance suffers from inaction; if you move too far outside of your comfort zone，you melt down from stress. Stepping outside of your comfort zone makes you better, and it doesn’t have to be something as extreme as climbing Mount Everest. It’s the everyday’s challenges that push your boundaries the most, none of which require a flight to Nepal. Just embrace these challenges since no one has ever accomplished anything remarkable when comfortable.