Which one of the following exhibits enantiomerism?

Assume the aerodynamic drag force on a car is proportional to its speed. If the power output from the engine is doubled,then the maximum speed of the car:

An image formed by a concave mirror of focal length $6 \ cm$ is $3$ times the size of the object. What is the distance of the object from the mirror in $cm$?

In which plant group are the sporophylls arranged spirally along an axis to form distinct compact structures called strobili or cones?

$A$ force $\overrightarrow F = 6\hat i + 2\hat j - 3\hat k$ acts on a particle,causing a displacement of $\overrightarrow s = 2\hat i - 3\hat j + x\hat k$. If the work done is zero,find the value of $x$.

The efficiency of a Carnot engine is $50\%$ and the temperature of the sink is $500\,K$. If the temperature of the source is kept constant and its efficiency is raised to $60\%$,then the required temperature of the sink will be .... $K$.