Is the following Python code valid?

a={3,4,{7,5}}
print(a[2][0])

Which of the following data types in Python is mutable and does not allow duplicate elements?

A. Set

B. List

C. Tuple

D. Dictionary

How can you add an element to a set in Python?

A. add()

B. insert()

C. append()

D. extend()

Which method is used to remove an element from a set?

A. remove()

B. delete()

C. discard()

D. pop()

What does the len() function do when applied to a set or tuple?

A. Returns the number of elements in the set or tuple

B. Converts the set or tuple to lowercase

C. Converts the set or tuple to uppercase

D. Removes the last element