mitochondrial inheritance questions

[geneticsquest]

Information that pertains to questions below:
4 siblings (parents history unknown except that they were first cousins) all have hearing loss. All are married with unrelated spouses with no hearing loss.
Oldest (Female)
-has 4 female children with no hearing loss
2nd Oldest (Female)
-has 2 male children with no hearing loss
-1 female child with hearing loss
3rd sibling (Male)
-2 male children and 1 female child with no hearing loss
Youngest (Female)
-3 male children and 1 female child all with hearing loss

portion of the normal sequence of the tRNA gene: GCTTTGGGGGGTTC
portion of the mutant sequence of the tRNA gene: GCTTTGGGGGGGTTC

Question #1) Only one of the siblings (a female) had 100 percent mutant mitochondrial DNA. Based on the information above, which of the siblings had 100 percent mutant mitochondrial DNA?
*I chose the youngest- because of how many offspring were affected ????

Question #2) Why do some of the female siblings with hearing loss have children who do not have hearing loss? (mutations in mitochondrial DNA are usually transmitted in a uniparental-maternal fashion in mammals)
a: the females who had offspring without hearing loss were heteroplasmic
b: there are two mitochondrial serine tRNA genes. The other tRNA gene sometimes compensates for the loss of function in the mutant tRNA gene.
c: Biparental inheritance of mitochondrial DNA caused some of the offspring to escape hearing loss.
d: those with hearing loss were segregational petites, whereas those without hearing loss were suppressional petites.
*I chose d because the suppressional petites would account for no hearing loss??

Question #3) Which of the following is the most probable cause of this mutation?
a: slippage during DNA replication
b: tautomeric shift
c: spontaneous deamination
d: SOS mutagenesis
*I chose a because that one seems to be the most likely cause

Thanks in advance for your help!!

[JackBean]

1) yeah, the youngest seem so

2) a could be true
I don't know, what is the cause, so b could be true, theoretically too
c could account as well, because some male mitochondria are transfered as well (but much less than female's;)