Why does loss of telomerase activity in humans lead to severe phenotypes in the first generation,while loss of telomerase activity in laboratory mice takes several generations to exhibit any sort of phenotype?

A)Mouse cells have shorter telomeres.
B)Human cells have shorter telomeres.
C)Mouse cells express higher levels of hTERT.
D)Human cells express higher levels of hTERT.
E)The length of mouse telomeres does not change over time.