Read the passage.
around the mid-17th century, robert hooke observed part of a cork tree with a simple microscope. he realized that the cork consisted of small, box-like parts. he called these box-like parts cells. in the late 17th century, one-celled organisms were first observed with a microscope. until that time, nobody knew that microscopic life forms existed.
in the 19th century, improved microscopes and processes were used. this led to the discovery that animal tissues were also made of cells. in addition, 19th-century scientists observed how new cells formed. they realized that cells always form from cells that already exist. scientists also discovered that living cells could carry out certain chemical reactions.
which option correctly explains how the cell theory was formed?
a) scientists formed the cell theory in the 17th century but stopped using it until new discoveries were made in the 19th century.
b)scientists’ understanding of cells continually improved as the results of studies built upon each other over time and formed the cell theory.
c)once the original facts were discovered, the only part of the cell theory that could change was its structure.
d)scientists discovered cells in the 17th century and created the cell theory in the exact form in which it is seen today.