Why does the blood not become acidic during the transport of $CO_2$?

Dissociation of $CO_{2}$ from carbamino haemoglobin takes place when

The presence of $CO_2$ in blood will lower $pH$ because $CO_2$ combines with . . . . . . ,with the rate of reaction increased by . . . . . . .

If $ppCO_2$ is high and pH is low,then it will result in:
$i$. increase in affinity of haemoglobin for $O_2$
$ii$. decrease in affinity of haemoglobin for $O_2$
$iii$. oxygen dissociation curve shifts to left
$iv$. causes release of $O_2$ for diffusion into tissue cells
$v$. $H^+$ and $HCO_3^-$ ions combine to form carbonic acid

Which of the following factors affect the binding of oxygen with hemoglobin?

Which of the following combines irreversibly with blood haemoglobin?