Hannah just finished building a house of cards that stands four stories high. she is worried that it will fall down. which of the following statements must be true?
if no unbalanced force acts upon the house of cards, then it will remain standing forever.
as long as nobody touches the house of cards, it will remain standing.
if hannah adds another card to the house of cards, then it will fall down.
a house of cards is too fragile and must fall down eventually due to gravity.

If no unbalanced force acts upon the house of cards, then it will remain standing forever.

Explanation:

From Newton's first law we understand that for an object to change its state of rest or motion, an external unbalanced force must act on it.

In the given situation, the house of cards is at rest and would remain at rest until and unless an unbalanced external force acts on it. Thus, till then it would remain standing still.

i don't know that i agree with that but what's the question ?

there are no contact forces mentioned on the list of choices that you've provided.