How many high-class products are sold by preferred vendors?
preferred vendor refers to PreferredVendorStatus = 1; high class refers to Class = 'H'
SELECT COUNT(T2.Name) FROM ProductVendor AS T1 INNER JOIN Product AS T2 USING (ProductID) INNER JOIN Vendor AS T3 ON T1.BusinessEntityID = T3.BusinessEntityID WHERE T3.PreferredVendorStatus = 1 AND T2.Class = 'M'