请以资深高中物理教师的口吻和水平，详细解答这道电磁振荡题目，并给出具体的思路分析。---**Question Stem:**
9. 如图甲所示电路中，电流 i (以图甲中电流方向为正) 随时间 t 的变化规律如图乙所示的正弦曲线，下列说法正确的是 ( )
**Translation of Question Stem:**
9. In the circuit shown in Figure 甲, the change rule of the current i (taking the direction of the current in Figure 甲 as positive) with time t is shown as the sinusoidal curve in Figure 乙. Which of the following statements is correct?
**Options:**
A. 0 ~ $t_1$ 内, 电容器 C 正在充电
Translation: A. In the range 0 ~ $t_1$, capacitor C is charging.
B. $t_1$ ~ $t_2$ 内, 电容器 C 上极板带负电
Translation: B. In the range $t_1$ ~ $t_2$, the upper plate of capacitor C is negatively charged.
C. $t_2$ ~ $t_3$ 内, 自感电动势逐渐增大
Translation: C. In the range $t_2$ ~ $t_3$, the self-induced electromotive force is gradually increasing.
D. $t_3$ ~ $t_4$ 内, 电场能转化为磁场能
Translation: D. In the range $t_3$ ~ $t_4$, electric field energy is converted into magnetic field energy.
**Diagram Description:**
**Figure 甲 (Jia):**
* Type: Circuit Diagram
* Components: A capacitor labeled "C" and an inductor labeled "L" connected in series.
* Connection: The capacitor and inductor are connected to form a closed loop (LC circuit).
* Current Direction: An arrow labeled "i" points clockwise, indicating the positive direction of current. The upper plate of the capacitor is connected to the left of the inductor.
**Figure 乙 (Yi):**
* Type: Line Chart (Graph)
* Axes:
* Horizontal Axis (X-axis): Time (t)
* Vertical Axis (Y-axis): Current (i)
* Curve: A sinusoidal curve representing the current i as a function of time t.
* Origin: Labeled "O" at (0,0).
* Key Points/Times:
* $t=0$: i = 0, slope is positive (increasing).
* $t=t_1$: i is maximum positive, slope is zero. (Peak of the first positive half-cycle)
* $t=t_2$: i = 0, slope is negative (decreasing). (Zero crossing after positive peak)
* $t=t_3$: i is maximum negative, slope is zero. (Trough of the first negative half-cycle)
* $t=t_4$: i = 0, slope is positive (increasing). (Zero crossing after negative trough, completing one cycle)
* The curve represents one full cycle of oscillation of the current in the LC circuit.