Directions: The given sentence has four underlined words or phrases, which are marked as (1), (2), (3) and (4). Identify the one underlined word or phrase that must be changed in order for the sentence to be correct.

Although meteorologists (1) begun to unravel (2) the mysteries of frontal systems and (3) even tornadoes, thunderstorms have stubbornly clung to (4) their secrets.