Suppose a block with mass 4kg is placed on a horizontal table. Ryan tried to push the block to right with force 4N but failed. He then tried to push with 6N and 8N, both didn\textquotesingle t work. Lastly, Ryan pushed with 8.1N force, and the block finally moved. What is the friction acting on the block when Ryan pushes with 4N, 6N, and 8N, separately?  $\textasciitilde$  $\textasciitilde$ 
选项为:A:4N, 6N, 8N, all to left  B:8N, 8N, 8N, all to left  C:4N, 6N, 8N, all to right  D:8.1N, 8.1N, 8.1N, all to right  请回答其中一个选项.