Say, "Yes. What should I deprioritise?" Saying no to a senior leader at work is almost unthinkable, even laughable, for many people. However, when saying yes is going to compromise your ability to make the highest level of contribution to your work, it is also your obligation. In this case it is not only reasonable to say no, it is essential. One effective way to do that is to remind your superiors what you would be neglecting if you said yes and force them to grapple with the trade-off.