What is the output of the following?x = ['ab', 'cd']for i in x:    x.append(i.upper())print(x)

The code you've posted will result in an infinite loop.

Here's why:

1. You're iterating over the list x which initially contains two elements: 'ab' and 'cd'.
2. Inside the loop, you're appending to the same list (x) you're iterating over. You're adding the uppercase version of each item to the end of the list.
3. Because you're adding to the list during iteration, the loop will never reach the end of the list, resulting in an infinite loop.

In general, it's not a good idea to modify a list (or any iterable) while iterating over it. If you need to create a new list that contains the original elements and the uppercase versions, you could create a copy of the original list to iterate over.