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Here is a quick way of making grass. You need to have at least Cinema 4D Release 9.5 and the Hair plug-in to follow this tutorial. But similar effects can also be created with the free Hair Department.
Making Grass
Create a Plane object and press the Add Hair button. This created a new Hair object and a default hair texture will be assigned to it. Leave all hair settings to default for the moment.
Make sure the Hair object is activated and select the hair scale tool (Hair > Tools > Scale). Use this tool to bring the future grass to the desired length (either by using the tool controls in the attributes manger or by clicking and dragging in the viewport).
Now select the brush tool (Hair > Tools > Brush). Adjust the radius and bring in some variation in the grass. Make sure to rotate the viewport and work on all sites of the object.
This is it for the 'modeling' :) Now select the hair material. Three channels are activated by default: Color, Specular and Thickness. This is what you should see when you do a quick render of your scene.