The $$stretched$-$S$$ pattern observed during the project life cycle, for the most part, is a result of the $\_\_\_\_\_\_\_\_\_.$

a$.$

principle that work expands to fill the allotted time

b$.$

changing attitudes of the people who work in the project.

c$.$

changing levels of resources used during successive stages of the project life cycle

d$.$

difficulty in performing the individual activities of a project.