Alleles can contain the code for:
a. only recessive genes.
b. only the same version of a gene.
c. different genes but of a similar type.
d. the same gene, but different versions.

D. the same gene, but different versions.

Explanation:

Because an allele is an alternative form of a gene, meaning two alleles codes for a gene.

One can be dorminant and the other recessive or both dorminant and or both recessive.

Hope it helps.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.