Physics: Help with Work(concept) When is it applied? 10PTS!!!

I'm having a hard time telling when a question actually has work in it or not. For example, a man pushs against a wall with a force of 25N for 15s, what the work done by the student on the wall:

If he doesn't accelerate it, isn't there 0 work done? Also which of the following does not have work done to the object:

A) an object held 1.5m above the ground
B) an object moves at a constant velocity(.5m/s) horizontally on a frictionless surface
C) an object falls freely from 1.5m to the floor

--- I think theres work on C because it accelerates but what about a or b is work done on those?

Last question: A physics student explains to his friend that work is done on objects for the following reasons, which are correct:
A) to over come gravity
B) to accelerate the object
C) to overcome friction

I dont so much need the answer as the concept of when work is applied and when it is not. Is work only applied if it accelerates or decelerates? Any help would be great!

My physics may be a bit rusty (it's been two years since I've done any) but I was pretty good at it, and here's my explanation:

Work = Force * Distance.
So whenever you are given these appropriate values (force and distance, or two of the three) then you know you are working with a work problem. Or, if the problem gives you values where you can actually hammer out yourself to getting two of the three components. That's when you know you should do work. And of course, if it asks you for the work, then obviously you have to calculate it. Here's my explanation for the other questions you have:

Student does force of 25N for 15s on the wall:
- I feel that this is a trick question and that there is no work done. From what I remember Work = Force * Distance. If you push against a wall... there's no distance... 'cause it doesn't move... and time shouldn't be a factor.