Which two numbers (not digits) and which two signs should be interchanged to yield the value 'zero' for the following e×pression?
13 + 65 ÷ 2 × 8 - 21
1
− and ÷; 8 and 13
2
+ and ÷; 13 and 21
3
× and −; 21 and 65
4
+ and ÷; 13 and 65