$n$ conducting wires of same dimensions but having resistivities $1, 2, 3, . . . , n$ are connected in series. The equivalent resistivity of the combination is

The periodic time of a particle doing simple harmonic motion is $4 \ s$. The time taken by it to go from its mean position to half the maximum displacement (amplitude) is

The full form of $NAD$ is

Which of the following substances combines with hemoglobin in the blood to form carboxyhemoglobin?

Arrange the following compounds in decreasing order of reactivity towards nucleophilic addition reactions:
$(I) HCHO$
$(II) CH_3CHO$
$(III) CH_3COCH_3$
$(IV) CH_3CH_2COCH_2CH_3$

Box $A$ contains $2$ black and $3$ red balls, while Box $B$ contains $3$ black and $4$ red balls. Out of these two boxes, one is selected at random; and the probability of choosing Box $A$ is double that of Box $B$. If a red ball is drawn from the selected box, then the probability that it has come from Box $B$ is: