Which Transact-SQL query should you use?

Your database contains a table named Purchases. The table includes a DATETIME column
named PurchaseTime that stores the date and time each purchase is made. There is a nonclustered index on the PurchaseTime column. The business team wants a report that
displays the total number of purchases made on the current day. You need to write a query
that will return the correct results in the most efficient manner.
Which Transact-SQL query should you use?

A.
SELECT COUNT(*)
FROM Purchases
WHERE PurchaseTime = CONVERT(DATE, GETDATE())

B.
SELECT COUNT(*)
FROM Purchases
WHERE PurchaseTime = GETDATE()

C.
SELECT COUNT(*)
FROM Purchases
WHERE CONVERT(VARCHAR, PurchaseTime, 112) = CONVERT(VARCHAR,
GETDATE(), 112)

D.
SELECT COUNT(*)
FROM Purchases
WHERE PurchaseTime >= CONVERT(DATE, GETDATE())
AND PurchaseTime < DATEADD(DAY, 1, CONVERT(DATE, GETDATE()))

Explanation:
http://technet.microsoft.com/en-us/library/ms181034.aspx