Rhubarb Upside-Down Cake
["5 c. fresh or frozen rhubarb, cut into 1/2-inch pieces", "1 (6 oz.) pkg. strawberry gelatin", "1/2 c. sugar", "2 c. miniature marshmallows", "1 (18 1/4 oz.) pkg. white or yellow cake mix, prepared to pkg. directions"]
["Place rhubarb in a greased 13 x 9-inch pan.", "Sprinkle with gelatin, sugar and marshmallows.", "Pour cake batter over marshmallows.", "Bake at 350\u00b0 for 50 minutes or until toothpick inserted in the middle comes out clean.", "Invert cake onto a serving plate.", "Serve with whipped topping if desired."]
["frozen rhubarb", "strawberry gelatin", "sugar", "marshmallows", "white"]