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.

[Q]: Yellow fever begins suddenly after an incubation period of three to ____ days.
[A]: five


[Q]: Variable stars fall into ____ major physical classifications.
[A]: four


[Q]: Language has ____ forms, written and spoken.
[A]:
two