Using Big$-$O notation, indicate the time requirement of each of the following tasks in the worst case.

After arriving at a party, you shake hands with each person there

$O(n)$

Each person in a room shakes hands with everyone else in the room

$O(n^{****}2)$

After entering an elevator, you press a button to choose a floor

$O(1)$

You ride the elevator from the ground floor up to the nth floor

$O(n)$