You have two hourglasses. One measures 7 minutes, and the other measures 4 minutes. How can you measure exactly 9 minutes using these two hourglasses?
a) Start both hourglasses simultaneously, and when the 4-minute hourglass runs out, flip it again.
b) Start the 7-minute hourglass. When it runs out, start the 4-minute hourglass.
c) Start both hourglasses simultaneously, and when the 7-minute hourglass runs out, flip it again.
d) Start the 4-minute hourglass twice.
