Which behavior is an instinct? A. An elk gets used to hikers and stops running away.
B. A pet dog sits and waits for a treat after its daily walk.
C. A pigeon visits a local park to look for dropped crumbs.
D. A robin builds a nest using mud, grass, and small twigs.

D

Explanation:

A robin building a nest is not learned or taught like the others. It is just instinctive for robins to build nests.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.