Down's syndrome is an example of

The number of chromosomes in Klinefelter's syndrome is

Given below are two statements :
Statement $I$ : Down's syndrome is caused by the absence of one of the $X$-chromosomes.
Statement $II$ : Turner's syndrome is caused by the presence of an additional copy of the chromosomes.
In the light of the above statements, choose the correct answer from the options given below :

If there is an addition of a single chromosome to the total set of chromosomes,it is called . . . . . . .

If the haploid chromosome number in a cell is $12$,then the monosomic number will be:

Down's syndrome occurs in the $21^{st}$ pair of chromosomes due to the presence of an additional chromosome.