Two capacitors $C_1$ and $C_2$ are charged to $120 \ V$ and $200 \ V$ respectively. It is found that by connecting them together,the potential on each can be made zero. Then:

The equation of the circle with radius $5$ and touching the coordinate axes in the third quadrant is

The element with atomic number $12$ belongs to ......... group and ......... period.

$\Delta \lambda$ is the difference between the wavelength of the $K_{\alpha}$ line and the minimum wavelength of the continuous $X$-ray spectrum when the $X$-ray tube is operated at a voltage $V$. If the operating voltage is changed to $V/3$,then the above difference is $\Delta \lambda^{\prime}$. Then:

Which of the following acids has the lowest $pK_a$ value?

Which of the following lubricates ligaments or tendons and is an important constituent of synovial fluid of bones?