Which type of ladder has flat steps, hinged back, self-supporting, and non-adjustable?

 Step Ladders

In general ladders are with inclined or vertical set of steps or rungs between two upright lengths of metal or wood. Ladders used for climbing up and down.

Step ladders :

It is a self supporting portable ladder, most commonly used in industries.  Self-supporting means it does not need any support to lean.  So this ladder can be used any where in the rooms. For example middle of the room  where support is not available. Also this ladder is non-adjustable flat steps and hinged back.  

work: w = fd (given)

work done by chet   =50*1 = 50 j

work done by mika =40*2 = 80 j

work done by sara =30*1.5 = 45 j

work done by bill =60*0.5 = 30 j

students in order from the greatest amount of work done to the least is mika> chet> sara> bill

hence option (b) is correct.

d) physical means can be used to separate the components of air.