Abacus, Inc. forms a corporation, Sirius, Inc., by transferring 18 percent of Abacus's stock to it for 100 percent of the stock in Sirius. Sirius, Inc. acquires 90 percent of the stock of Tyrol, Inc. for its stock in Abacus, whereupon Sirius is merged into Tyrol, the latter surviving. These transactions may best be described as

a.A Section 351 transfer, followed by a Type B reorganization

b.A spin-off

c.The purchase of a subsidiary corporation

d.A reverse triangular merger