While dissecting an earthworm, you notice a closed tube that sits atop the digestive tract. Within each segment, this dorsal tube has a pair of branches that extend to connect with a similar ventral tube. What are you looking at?
A) a closed circulatory system
B) a nerve cord
C) an excretory system
D) an open circulatory system

Explanation:

1. The  circulatory system; in earthworm is a closed type . (blood is contained in vessels) .

2. An earthworm circulates blood  through vessels only.

3. There are 3 main vessels  in earthworm that supply the blood to organs  and whole body.  

These vessels are called as  i)  dorsal blood vessels,ii) the aortic arches and iii) ventral blood vessels.  

4. The aortic arches of  earthworm function just like as human heart.

stratosphere: as the altitude increases, the air temperature increases. the stratosphere has a layer of ozone, called the ozone layer. this layer absorbs most of the ultraviolet radiation from sunlight. this results in the stratosphere being warmer.