Which of the following reactions will not result in the formation of carbon-carbon bonds?

$A$ piece of iron is heated in a flame. It first becomes dull red, then becomes reddish yellow, and finally turns to white hot. The correct explanation for the above observation is possible by using:

In a $\triangle ABC$,the expression $\frac{(a+b+c)(b+c-a)(c+a-b)(a+b-c)}{4b^2c^2}$ is equal to:

At a certain place,the horizontal component of Earth's magnetic field is $\sqrt{3}$ times the vertical component. The angle of dip at that place is....$^o$

The combined equation of the bisectors of the angles between the lines joining the origin to the points of intersection of the curve $x^2+y^2+xy+x+3y+1=0$ and the line $x+y+2=0$ is

The locus of the centre of a circle which passes through the origin and cuts off a chord of length $4$ units from the line $x=3$ is