3.1-3 为什么“算法A的运行时间至少是O(n^2)”这一表述是无意义的

Let the running time be T(n). T(n) >= O(n^2) means that T(n)>= f (n) for some
function f (n) in the set O(n^2). This statement holds for any running time T(n),
since the function g(n) = 0 for all n is in O(n^2), and running times are always
nonnegative. Thus, the statement tells us nothing about the running time.