Which of the following organisms has a cell wall composed of chitin?

What should be the angular speed of the earth,so that a body lying on the equator may appear weightless? $(g = 10\,m/s^2, R = 6400\,km)$

Match the following columns:

Column-$I$	Column-$II$
$(A)$ Blue-green algae	$(I)$ Spirogyra
$(B)$ Unicellular algae	$(II)$ Nostoc
$(C)$ Filamentous algae	$(III)$ Volvox
$(D)$ Colonial algae	$(IV)$ Chlamydomonas

Which one of the following is formed apart from sodium chloride when chlorine reacts with hot concentrated sodium hydroxide?

The first line in the Lyman series has wavelength $\lambda$. The wavelength of the first line in the Balmer series is

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