Combine sugar, celery seed, salt and mustard,Gradually stir in water and cider vinegar until smooth in saucepan over medium heat. In separate container mix a small amount of water with arrowroot until mixture is smooth, stir in gradually to above ingredients in saucepan until mixture thickens. Refrigerate until chilled, about 3 hours. Mix in salad dressing and yogurt. Combine cabbage and apples in serving bowl and pour on dressing. Toss lightly to coat. Serve immediately or refrigerate.