The first suggestion that chromosomes may physically exchange segments came from the microscopic identification of
synapsis
sister chromatids
chiasmata
alleles

chiasmata

Explanation:

Chiasmata is the meeting point of homologous chromatids regarding meiotic cell division.

This structure conforms to homologous chromosomes when, in meiosis, part of the arm of each chromosome breaks and is recomposed into its homologous. This phenomenon, which is extremely important for evolution, is called chromosomal permutation or crossing-over. This phenomenon is associated with gene linkage, that is, linkage, which provides greater genetic variability and different gametic percentages than would be found according to Mendel's second law.

Based on this, we can conclude that the first suggestion that chromosomes could physically exchange segments came from the microscopic identification of chiasmata.

b. the carbon cycle ensures living things have a source of hydrogen

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.