Solution:

(i) We use if and only if, in the bi conditional statement.

: The unit digit of an integer is zero.

: It is divisible by 5.

Therefore,

Unit digit of an integer is zero if and only if it’s divisible by 5.

Solution:

(ii) We use if and only if, in the bi conditional statement.

: A natural number is odd.

: Natural number isn’t divisible by 2.

Therefore,

A natural number is odd if and only if it isn’t divisible by 2.