Which of the following statements is correct related to half adiler. Which can add two one digit binary numbers?

A$.$ It is possible to add $01$ and $11$ by uning half odder.

B$.$ Sum $($S$)$ and Carry $($C$)$ outputs can be equal to $0$ at the same time.

C$.$ It is not possitle to add $1$ and $0$ by using half adder.

D$.$ Sum $($S$)$ and Carry $($C$)$ outputs can be equal to $1$ at the same time.

B$.$ None of these.

$20.$ Which of the following statements is correct related to full adder?

A$.$ Without a special treatment for a previous possible carry, addition of multi$-$digit binary numbers using full adder may give wrong results.

B$.$ One can not add $1$ and $0$ together with a previous carry by using a full adder.

C$.$ One can add octal numbers $7$ and $3$ by using two full adders.

D$.$ It is possible to add decimal numbers $9$ and $8$ by using a full adder.

E$.$ None of these.

$21.$ Which of the following statements is correct related to carry lookahead generator?

A$.$ All of them.

B$.$ It is necessary related to timing problem.

C$.$ All carries are obtained from an initial carry and outputs of half adders, i$.$e$.P_i$ and $G_i.$

D$.$ It is more complicated than usage of only full adders.

E$.$ All carries are generated through two levels of gates.

A four$-$bit binary adder$-$subtractor circuit can be seen at the figure. Following $2$ questions are related to it$.$

$22.$ Which of the followings is true related to four$-$bit binary adder$-$subtractor which is available in the figure?

A$.$ It can not do subtraction.

B$.$ Exclusive$-$OR gates do all necessary things during the calculation of the $2\text{'}$s complement.

C$.$ It does addition when $M=1.$

D$.$ It does subtraction $M=1$ by using $2\text{'}$s complement

E$.$ None of them.