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 reorganizationb.A spin$-$offc.The purchase of a subsidiary corporationd.A reverse triangular merger