Dictionaries - General Questions

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?

d1 = {'a': 1}
d2 = {'b': 2, 'a': 9}
d1.update(d2)
print(d1)

What is the output of the following code?

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

Which of these cannot be a dictionary key?