Questions

It is ABACUS.

Words formed are as below 

BUS
CAB
CUB
US

Black Mamba

This acceleeration denoted by letter g varies from place to place. Unless stated otherwise, it can be taken 9.8 m/s2.

To display current date and time in MS Excel, we use Now() method. 

When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state. System is in safe state if there exists a safe sequence of all processes. Deadlock Avoidance : ensure that a system will never enter an unsafe state.

Sieve.

A utensil consisting of a wire or plastic mesh held in a frame, used for straining solids from liquids.

An organelle called Eyespot allows some protists to detect light. It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light (positive phototaxis) or away from the light (negative phototaxis).

Eyespot is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids.

Eyespot-mediated light perception helps the cells in finding an environment with optimal light conditions for photosynthesis. Eyespots are the simplest and most common "eyes" found in nature, composed of photoreceptors and areas of bright orange-red pigment granules.