Concept:The correct word must show that the weight is more than what can be lifted.Explanation:The phrase “too heavy for us to lift” means the sack exceeds our ability to lift it.The word too is used before an adjective with an infinitive to show an excessive degree.very and quite only intensify the adjective but do not express excess.so would require a different clause, such as “so heavy that…”Therefore, too is the only option that completes the meaning correctly.Answer:too