TASK DEFINITION: Given a sentence, fill out the missing word with a 'no' or a number (between zero and ten). You should write the numbers with english alphabet, like: four instead of 4.
PROBLEM: Copper has ____ naturally occurring isotopes.

SOLUTION: two

PROBLEM: Medial moraine is formed from ____ lateral moraines.

SOLUTION: two

PROBLEM: Refraction is caused by ____ effects.

SOLUTION:
two