The entire circulatory system is lined with a .
A. conjoined plasma membrane
B. one-cell-thick layer of flattened cells
C. single long conducting tube
D. wall of cilia
E. one-cell-thick layer of rounded cells

Option B.

One cell thick layer of flattened cells.

Explanation:

The circulatory system is a system or network in the body that consist of blood, blood vessels i.e arteries and veins , microvessels, lymph vessels and the heart. They are lined by simple squamous epithelium tissue, these tissue is one cell thick layer of flattened cells. These type of cells are flat with flattened nuclei and are permeable, that is they allow the passage of small molecules through filtration and diffusion. The circulatory system help to circulate blood through out the body from the heart.

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ya the other dude is right