用适当的词填空

Nowadays we can <t > time accurately but before the invention of clocks it was very <d > One way people told the time was by <u > sundials but it was impossible to tell the time on <c > days or at night

tell, difficult, using, cloudy

Nowadays we can <tell > time accurately but before the invention of clocks it was very <difficult > One way people told the time was by <using > sundials but it was impossible to tell the time on <common > days or at night