The most important ore of tin is:

Four charges are placed at the circumference of a dial clock as shown in the figure. If the clock has only an hour hand, then the resultant force on a charge $q_0$ placed at the center points in the direction which shows the time as: (in $.30$)

The reason for the double helical structure of $DNA$ is the operation of

Brush bordered epithelium is found in

Which one of the following statements regarding helium is incorrect?

If the area of the triangle formed by the pair of lines $8x^2 - 6xy + y^2 = 0$ and the line $2x + 3y = a$ is $7$,then $a$ is equal to