Company A launchs a product. The product enjoys steep sales growth during the launch phase which lasts $6$ months and then that growth stabilizes at the high level that it reached during the launch phase for another $18$ months. With increasing compettive rivalry, the product's sales decline for another $6$ months before it is finally phased out by Company A$.$ The time that Company A is able to generate economic returns from the product is

$30$

$42$

$18$

$24$