7. (1987 American High School Mathematics Examination) Randomly decompose 2.5 into the sum of two non-negative numbers, such as $2.5=$ $2.143+0.357$ or $2.5=\sqrt{3}+(2.5-\sqrt{3})$, then change each number to the nearest integer, such as the first example's $2.143,0.357$ being changed to 2,0, and the second example's $\sqrt{3},(2.5-\sqrt{3})$ being changed to 2,1, then the probability that the sum of the final two integers is 3 is ( ).
A. $\frac{1}{4}$
B. $\frac{2}{5}$
C. $\frac{1}{2}$
D. $\frac{3}{5}$
E. $\frac{3}{4}$
7. B. Reason: For 2.5, according to the problem, decompose (into two non-negative numbers) and take the integer part (write these two numbers as the closest integers) of the two numbers, which can only be the following few cases:
$$
0,2 ; 1,1 ; 2,0 ; 2,1 ; 1,2 \text {. }
$$

Among these, there are 2 cases where the sum is 3, so the probability is $\frac{2}{5}$.
Or for the decomposition of $x+y=2.5$, it can be represented by the following diagram (each vertical line intersects the two number axes at points representing a decomposition), the circled numbers are the integers taken when the numbers in the interval segments (each with five segments) are rounded.

Obviously, the decompositions that fit the problem are only 2 segments, so the probability is $\frac{2}{5}$.