Which statement accomplishes all the required tasks?

The CUSTOMERS table has the following structure:

You need to write a query that does the following task:
* Display the first name and tax amount of the customers. Tax is 5% of their credit limit
* Only those customers whose income level has a value should be considered
* Customers whose tax amount is null should not be considered
Which statement accomplishes all the required tasks?

A.
SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT FROM customers
WHERE cust_income_level IS NOT NULL AND
tax_amount IS NOT NULL;

B.
SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT FROM customers
WHERE cust_income_level IS NOT NULL AND
cust_credit_limit IS NOT NULL;

C.
SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT FROM customers
WHERE cust_income_level <> NULL AND
tax_amount <> NULL;

D.
SELECT cust_first_name, cust_credit_limit * .05 AS TAX_AMOUNT FROM customers
WHERE (cust_income_level,tax_amount) IS NOT NULL;