The calorie expenditure for road cycling and mountain biking can vary based on factors such as intensity, terrain, and duration. Generally, road cycling on flat terrain at a moderate pace can burn around 400-600 calories per hour, while mountain biking on varied terrain with hills and obstacles can burn around 500-800 calories per hour. The higher calorie expenditure in mountain biking is due to the increased effort required to navigate the challenging terrain and obstacles. Overall, mountain biking tends to result in a higher calorie expenditure compared to road cycling. **

To send an envelope with a prepaid return envelope, first place the items you want to send in the main envelope. Then, insert the prepaid return envelope inside the main envelope. Seal the main envelope and affix the necessary postage. Finally, address the main envelope to the recipient and it's ready to be mailed. When the recipient receives the main envelope, they can use the prepaid return envelope to send items back to you without having to worry about postage. **

"With or without envelope?" is a question commonly asked when sending a letter or card. It refers to whether the sender wants the item to be placed in an envelope before being sent. If the sender chooses "with envelope," the item will be enclosed in an envelope for protection and privacy. If the sender chooses "without envelope," the item will be sent as is, without any additional packaging. **

Whether or not you should stop mountain biking depends on your personal circumstances and reasons for considering stopping. If you are experiencing physical pain or injury from mountain biking, it may be wise to take a break and seek medical advice. If you are feeling burnt out or no longer enjoying the activity, it may be beneficial to explore other forms of exercise or outdoor activities. Ultimately, the decision to stop mountain biking should be based on your own well-being and enjoyment. **

The contents of the envelope can vary depending on the sender and the purpose of the mailing. It could contain a letter, a card, important documents, money, a gift card, or any other item that can fit inside. The only way to know for sure what is inside the envelope is to open it. **

A stamped envelope is a pre-paid envelope that already has postage affixed to it. This means that the sender does not need to add additional stamps to the envelope before mailing it. Stamped envelopes are convenient for sending mail as they eliminate the need to calculate postage costs and affix stamps separately. They are often available for purchase at post offices and can come in various sizes and designs. **

A prepaid envelope is an envelope that already has the postage paid for, so the sender does not need to affix additional stamps. This type of envelope is often used for business reply mail, where the recipient of the mail is responsible for the postage costs. Prepaid envelopes can also be used for sending documents or other items where the sender wants to ensure that the recipient does not need to pay for postage. **

I'm sorry, but as a language model AI, I am unable to physically see or determine if an envelope is pre-paid. You may need to check for any pre-paid markings or postage on the envelope itself to confirm if it is pre-paid. If there is no indication, you may need to ask the sender or the postal service for confirmation. **