Who has a "priority" in the following cases of competing creditors?

a. Two creditors can prove that they've a perfected security interest in a debtor's tractor trailer. As between the two, who has priority?

b. In 2008, a local business signed a security agreement granting a security interest to First Bank in several specified pieces of equipment and "all after acquired property" owned by the business. In 2010, the business purchased inventory and financed that purchase with the seller. Who has a priority regarding that purchased inventory between First Bank and the seller?

c. What if the purchase in b. above had been equipment, not inventory?