Preliminary information suggests that the effectiveness of COVID vaccines is different for those 65 years of age and over and those under 65 years of age. Knowing this, when vaccine manufacturers conduct an experiment, they should use a ...

	a.	Block design, with gender as the blocks
	b.	A block design with age as the blocks.
	c.	A stratified random sample design
	d.	Block design, with type of shot (new versus placebo) as the blocks.