基于initial语句产生普通时钟信号，parameter clk_period = 10;reg clk;initial begin clk = 0; ________________;end
选项：

A:always #(clk_period/2) clk = ~clk;
B:forever #(clk_period/2) clk = ~clk;
C:always #(clk_period) clk = ~clk;
D:forever #(clk_period) clk = ~clk