Problem 4 (Negative interest rates). (9 pts, 3 pts for each sub-problem) Previously, some countries such as Japan and Switzerland have negative interest rates. What is the impact (if any) of negative interest rates on: a) The put-call parity result for European options b) The result that American call options on non-dividend-paying stocks should never be exercised early c) The result that American put options on non-dividend-paying stocks should sometimes be exercised early. Assume that holding cash earning zero interest is not possible. (Hint: Use put-call parity for European options and the fact that American options are at least valuable as European options.)