Which of the following oxides is the anhydride of orthophosphoric acid $(H_3PO_4)$?

Which of the following reacts with benzene in the presence of anhydrous aluminum chloride to form acetophenone?

Among the following series of transition metal ions,the one where all metal ions have $3d^2$ electronic configuration is

Production of a human protein in bacteria by genetic engineering is possible because

The point $(3,-4)$ lies on both the circles $x^2+y^2-2x+8y+13=0$ and $x^2+y^2-4x+6y+11=0$. Then,the angle between the circles is

$P, Q, R$ and $S$ are four points with the position vectors $3i - 4j + 5k, 2i - j + 4k, -4i + 5j + k$ and $-3i + 4j + 3k$, respectively. Then, the line $PQ$ meets the line $RS$ at the point