Indicate the punctuation needed in the blank space in the following sentence. Many students have failed that professor's exams___ the most common reason is that students can't stay awake through all those boring lectures.

Practice this Upwork Skill Test. Freelancers take skill tests on Upwork to show their skills to potential clients and to make themselves more marketable. Upwork suggests you take as many tests as you want -  your skills are shown on your Upwork profile page.