Which of the following is true of kinetochores?

• They are the primary centrosome structures that maintain the attachment of the sister chromatids prior to mitosis.
• They are the sites at which microtubules attach to chromosomes.
• They are located at the center of the centrosome and organize tubulin into spindle fibers.
• They cross at the cell's equator and move against each other, causing the cell to elongate.