Which of the following statements are true?

(a) If a number is divisible by 3, it must be divisible by 9.

(b) If a number is divisible by 9, it must be divisible by 3.

(c) A number is divisible by 18, if it is divisible by both 3 and 6.

(d) If a number is divisible by 9 and 10 both, then it must be divisible by 90.

(e) If two numbers are co-primes, at least one of them must be prime.

(f) All numbers which are divisible by 4 must also be divisible by 8.

(g) All numbers which are divisible by 8 must also be divisible by 4.

(h) If a number exactly divides two numbers separately, it must exactly divide their sum.

(i) If a number exactly divides the sum of two numbers, it must exactly divide the two numbers separately.

(a) False
Explanation: 12 is divisible by 3 but is not divisible by 9.

(b) True
Explanation: As 9 is a product of 3 and 3. So, if a number is divisible by 9, it is also divisible by 3.

(c) False
Explanation: Since 42 is divisible by both 3 and 6 but is not divisible by 18.

(d) True
Explanation: As 9 and 10 are co-primes, so number divisible by both 9 and 10 is also divisible by their product. Therefore if a number is divisible by both 9 and 10 then it is divisible by 90.

(e) False
Explanation: 16 and 34 both are composite numbers and as they have 1 as the common factor they are also co-prime numbers. 

(f) False
Explanation: 28 is divisible by 4 but is not divisible by 8.

(g) True
Explanation: As 8 is the product of 2 and 4. So, if a number is divisible by 8, it will also be divisible by 2 and 4.

(h) True
Explanation: 3 divides 6 and 9. Then 3 also divides 15 (6 + 9 = 15).

(i) False
Explanation: 2 divides 14 but it does not divide 9 and 5.