Read the following statements and answer the question.
$(i)$ It is made of a single thin layer of flattened cells with irregular boundaries.
$(ii)$ They are found in the walls of blood vessels and air sacs of lungs.
$(iii)$ They are involved in functions like forming a diffusion boundary.
Which of the following characteristics of tissue is being described by the above statements?

• A
  Squamous epithelium
• B
  Columnar epithelium
• C
  Ciliated epithelium
• D
  Compound epithelium