Conditional civil legal act: A civil legal action that determines the conditions under which the effect of the action occurs or is eliminated is attached to the expression of intention. Which of the following is a conditional civil juristic act? ['A and B enter into a house lease contract, which stipulates that the contractor will terminate the contract as soon as the son of the lessor A gets married and needs to arrange a new house.', 'A said to B: "If my son is admitted to a prestigious university, I will invite you to dinner. ".', 'A and B signed a summer grain sales contract, which agreed to take effect in mid-November.', "A and B agree: If A helps B to fight B's enemies at night, B will pay A 1,000 yuan."]