Question

Which of the following is/are true?

• A. The sum of angles of any triangle is a constant and is equal to
  two right angles.
• B. Straight lines parallel to the same straight line are parallel to one another.
• C. All right angles are not equal to one another.
• D. There is a unique line that passes through a given point.

Answer 1

Answer: (A), (B)

The correct options are
A The sum of angles of any triangle is a constant and is equal to
two right angles.
B Straight lines parallel to the same straight line are parallel to one another.

Legendre's axiom says,
"The sum of angles of any triangle is a constant and is equal to
two right angles".

Proclus' axiom says,
"Straight lines parallel to the same straight line are parallel to one another".

Euclid's postulate says,
"All right angles are equal to one another".
$\therefore$ Saying they are not equal to one another is wrong.

Euclid's postulate says,
"There is a unique line that passes through two distinct points"
Infinite number of lines can pass through a given point.
$\therefore$ Saying only one line can pass through a given point is wrong.