What is a darker contor line usaly every fifth line?

Index contour.

Explanation: Index contours are those elevation line which are represented by a thick or darker line. It is because if we draw every elevation with a simple line, its not so reliable. So making every fifth contour line dark helps us in reading the map elevation clearly without any difficulties.

   This is very commonly used in the maps and are separated at equal heights.

the answers are as following:

1. c. double helix, nucleosome, coil, supercoil, chromosome

2. b. g2 phase, preparation for mitosis

3. d. interphase consists of the g1, s, and g2 phases

4. c. metaphase

5. a. chromosomes are duplicated before cell division so that each new daughter cell has a complete set