Arrange the following estuarine zones in decreasing order of salinity.
polyhaline
mesohaline
oligohaline
euhaline

Euhaline  ⇒  Polyhaline  ⇒  Mesohaline  ⇒  Oligohaline

Explanation:

Let's first define each of these levels:

- Polyhaline: a category of salinity that has medium-high levels of salt

- Mesohaline: a category of salinity that has medium levels of salt

- Oligohaline: a category of salinity that has low levels of salt

- Euthaline: a category of salinity that has high levels of salt

Now, we can order these in decreasing order of salinity:

Euhaline  ⇒  Polyhaline  ⇒  Mesohaline  ⇒  Oligohaline

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.