The United States has been prove

The United States has been proved to be laggard in the world, refusing to sign the protocol.
A、stupid
B、wise
C、rich
D、slow
【正确答案】：D
【名师解析】:题目中提到的单词"laggard"意味着落后或缓慢的。根据语境，美国拒绝签署协议，这表明它在这一领域的发展或行动上是缓慢的。因此，选项D "slow" 是最合适的选项，因为它与"laggard"的含义相符。其他选项A "stupid"、B "wise" 和 C "rich" 并不直接与"laggard"的含义相关联。