Different cell types and tissues express different sets of genes; for example, some genes are expressed primarily in the heart, others primarily in the brain, and still others primarily in the liver. This is possible because all of these cell types have . a. different sequence variants that control gene expression
b. different ordering of genes on chromosomes
c. different histone and DNA modifications
d. different sets of genes

d.Different sets of genes.

Explanation:

A cell normally only expresses a percentage of its genes and various cell types are created by the expression of distinct gene sets. In addition, in response to changes in their environment, cells can alter the pattern of genes they express, such as signaling from neighboring cells.

a) carbon, hydrogen

b) carbon, hydrogen, oxygen, nitrogen

c) carbon, hydrogen, oxygen, nitrogen, phosphorus

d) sugar

e) amino acid

f) nucleotide