Flagella differ from cilia in that flagella usually occur ______ while cilia usually occur ______.

• A. with a 9+0 microtubule arrangement; with a 9+2 microtubule arrangement
• B. singly or in pairs; in large numbers covering the cell surface
• C. in large numbers covering the cell surface; singly or in pairs
• D. only in prokaryotic cells; only in eukaryotic cells

Answer: (B)

Verified step by step guidance

1. Understand the structural and functional differences between flagella and cilia. Both are hair-like structures that extend from the surface of cells and are involved in movement, but they differ in their arrangement and occurrence.
2. Recall that flagella are typically longer and occur singly or in pairs. They are used for locomotion, such as in sperm cells or certain unicellular organisms like Euglena.
3. Cilia, on the other hand, are shorter and more numerous. They often cover the entire surface of a cell and are involved in coordinated movement, such as in the respiratory tract to move mucus or in Paramecium for locomotion.
4. Review the microtubule arrangement in both structures. Both flagella and cilia have a '9+2' arrangement of microtubules in eukaryotic cells, meaning 9 pairs of microtubules form a ring around 2 central microtubules. This is not a distinguishing feature between the two.
5. Eliminate incorrect options by focusing on the occurrence of flagella (singly or in pairs) and cilia (in large numbers covering the cell surface). This distinction helps identify the correct answer.