Directions: The series represents two pairs of words each of which are similarly related. Replace the question mark with the word that bears the same relationship with the word that precedes it as is the relation between the first two words in the series.

Bow : Rifle : : Hourglass : ?

A bow and a rifle are both weapons used to propel projectiles - one ancient (bow with arrows), one modern (rifle with bullets). Similarly, an hourglass and a chronometer are both timekeeping devices - one ancient (hourglass with sand), one modern (chronometer with precise mechanics). The relationship is 'ancient tool : modern equivalent doing the same job'.