未知题型 产生塑性变形必要的条件是（ ）。

单项选择题 Read the following definition paragraph and summarize what additional details (what, where, when, how, why, etc) are given to elaborate the definition. A reaction essay is a very common type of writing meant to describe your personal response or reaction to a piece of writing, a video or another piece of media or event. Reaction essays are meant not to convince others but rather to give your reaction and why you feel that way. Reaction essays are useful when dealing with current events, to describe how you feel about them and why. In order to create unity in a reaction essay, it is important to give the reader some background information about the prompt before you react to it. Background information varies based on the type of stimulus you are reacting to. If you are reacting to a visual prompt, the background information is usually a physical description of the item. If you are reacting to a written prompt, you can give a summary of it in the introduction. Present the most important elements and follow the order of the original. If you are reacting to a historical event or a theory, give factual information about it: specific dates, times, actions, and circumstances.

单项选择题 能够形成煤的植物是（）