Question

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?
A. A house of cards is too fragile and must fall down eventually due to gravity.
B. If no unbalanced force acts upon the house of cards, then it will remain standing forever.
C. If Hannah adds another card to the house of cards, then it will fall down.
D. As long as nobody touches the house of cards, it will remain standing