[单选题]
A spring of negligib
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hfidr4hc 3 w) *l;,u(fnq9fs ks and length of $l_o$ is attached to a fixed point. When the spring is extended by an external force $F$, the length of the spring increases to $l$. The graph shows how the length of the spring varies with the tension in spring. The tension in the spring is equal to where $k$ is a constant $k \Delta l$, and $\Delta l$ is the extension of the spring.
What are
the gradient and
y-intercept of the graph?