题干
So difficult ________ it to live in an English-speaking country that I determined to learn English well.
A：did I feel
B：I did feel
C：have I felt
D：I have felt
答案(点此获取答案解析）
A
，质量为m的小球静止在倾角为θ的光滑斜面上，则电场方向及大小下列判断正确的是
