The carbon atoms in the cellulose of a wood log originally came from absorbed by the plant.

Carbon dioxide

Explanation:

During photosynthesis, plants incorporate their only source of carbon through carbon dioxide (CO2) and together with water (H2O) and sunlight they synthesize glucose and oxygen.

6 CO2 + 6 H2O → C6H12O6 + 6 O2

Cellulose is a structural polymer formed by β-glucose (C6H10O5)n monomers arranged in an unbranched manner. This is synthesized in plants by the cellulose synthase complex (CSC). To do this the plant uses the glucose that was synthesized during photosynthesis.

So the source of the carbon atoms in cellulose came from the CO2.

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.