requirements, and the available capital funds over the next three years are summarized as follows.

$\backslash$table$[[,,,$Capital Requirements $($$$),],[\backslash$table$[[$Alternative$],[$Number$]],$Alternative,$\backslash$table$[[$Net Present$],[$Value $($$$)]],$Year $1,$Year $2,$Year $3],[1,$Limited warehouse expansion,$3,500,3,000,1,000,4,000],[2,$Extensive warehouse expansion,$6,500,2,500,3,500,3,500],[3,$Test market new product,$11,000,6,000,4,000,5,000],[4,$Advertising campaign,$3,500,2,000,1,500,1,800],[5,$Basic research,$8,500,5,000,1,000,4,000],[6,$Purchase new equipment,$2,500,1,000,500,900],[$Capital funds available,,,$10,500,7,000,8,750]]$

An integer programming model for maximizing the net present value $($in $ $)$ is developed. $($ Letting $x_i=\{[1if$ investment alternative $iis$ selected $],[0$ otherwise $i=1,2,3,4,5,6)]$

Management is concerned that at most three projects can be managed at one time. What additional constraint should be added to the integer programming model?