'Interact with one of the enrollment buttons on the page.
Keyword Arguments:
mode (str): Can be "audit" or "verified"
Raises:
ValueError'

def enroll(self, mode='audit'):

    if (mode == 'verified'):
        self.q(css='.contribution-option    >    input').first.click()
        self.q(css="input[name='verified_mode']").click()
        return PaymentAndVerificationFlow(self.browser, self._course_id).wait_for_page()
    elif (mode == 'audit'):
        self.q(css="input[name='audit_mode']").click()
        return DashboardPage(self.browser).wait_for_page()
    else:
        raise ValueError("Mode    must    be    either    'audit'    or    'verified'.")
