Dictionaries - General Questions

What is the output of the following code?

d = {1: 'one', 2: 'two'}
print({v: k for k, v in d.items()})

Which of these cannot be a dictionary key?

What is the output of the following code?

from collections import defaultdict
g = defaultdict(list)
g['a'].append(1)
print(dict(g))

What is the output of the following code?

d = {i: i * i for i in range(4)}
print(d[3])

What is the output of the following code?

d = {'a': 1, 'b': 2}
print(d.pop('a'), d)