HEL .
Which characteristic is only found among mammals?
lungs
vertebrate
endotherm
milk

The only characteristic found among mammals is that the females produce milk.

milk

Explanation:

mammals are the only group that produces true milk for their young

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.