Question: I-90 crosses the Continental Divide over a mountain pass that sits on the border between which two counties ?
Context: Rollins Pass, elevation 11660 ft , is a mountain pass in the Rocky Mountains of north-central Colorado in the United States.  The pass is located on and traverses the continental divide at the crest of the Front Range southwest of Boulder, and is at the boundaries of Grand, Gilpin, and Boulder counties.  The pass provides a route over the Continental Divide between the Atlantic Ocean watershed of South Boulder Creek (in the basin of the South Platte River) with the Pacific Ocean watershed of the Fraser River, a tributary of the Colorado River. Spring Creek Pass, elevation 10889 ft , is a mountain pass on the Continental Divide in the San Juan Mountains of Colorado.  The pass is traversed by State Highway 149 and the Colorado Trail.  Somewhat unusually for a pass on the Continental Divide, it is not the highest point on the highway in the vicinity; heading north from the pass, the road climbs over the considerably higher Slumgullion Summit before descending toward Lake City. The Continental Divide National Scenic Trail (in short Continental Divide Trail (CDT)) is a United States National Scenic Trail running 3,100 miles between Mexico and Canada.  It follows the Continental Divide of the Americas along the Rocky Mountains and traverses five U.S. states — Montana, Idaho, Wyoming, Colorado, and New Mexico.  In Montana it crosses Triple Divide Pass (near Triple Divide Peak which separates the Hudson Bay, Atlantic Ocean and Pacific Ocean drainages.) The trail is a combination of dedicated trails and small roads and considered 70% complete.  Portions designated as uncompleted must be traveled by roadwalking on dirt or paved roads.  This trail can be continued north into Canada to Kakwa Lake north of Jasper National Park by the Great Divide Trail, which is so far described only in a few books and carries no official Canadian status. MacDonald Pass, el.6312 ft , (also known as McDonald Pass) is a mountain pass on the continental divide west of Helena, Montana that is traversed by U.S. Route 12.  The pass is one of three passes (MacDonald, Mullan and Priest) used in the 1870s-80s for travel between Helena and Deer Lodge, Montana over the continental divide. Mullan Pass is a mountain pass in the western United States, in the Rocky Mountains of Montana.  It sits on the Continental Divide on the border between Powell and Lewis and Clark counties at 5902 ft above sea level.  In the Helena National Forest, the pass is Homestake Pass is a mountain pass in the Rocky Mountains of Montana in the United States.  It sits on the Continental Divide on the border between Jefferson County and Silver Bow County, six miles south-southeast of Butte in Beaverhead-Deerlodge National Forest at an elevation of 6329 ft . Interstate 90 (I-90) is a transcontinental freeway, and the longest Interstate Highway in the United States at 3,020.54 mi .  Its western terminus is in Seattle, at State Route 519 near Safeco Field and CenturyLink Field, and its eastern terminus is in Boston, at Route 1A near Logan International Airport.  I-90 crosses the Continental Divide over Homestake Pass just east of Butte, Montana.  East of the Wisconsin-Illinois border, much of I-90 follows several toll roads, many of which predate the Interstate Highway system.  The Jane Addams Memorial Tollway, Chicago Skyway, Indiana Toll Road, Ohio Turnpike, New York State Thruway, Massachusetts Turnpike, and the Ted Williams Tunnel are all toll roads used by I-90 in the eastern half of the country.  The Interstate is not tolled through some segments in downtown Chicago; Greater Cleveland, and the rest of Northeast Ohio; Pennsylvania; and through brief sections near Buffalo and Albany.  Erie, Pennsylvania; Madison, Wisconsin; Billings, Montana; and Spokane, Washington are other major cities on I-90. Lewis and Clark Pass, el.  6424 ft is a mountain pass on the continental divide in Montana.  It lies at the head of the drainages of the west flowing Blackfoot River and the east flowing Dearborn River.  The pass is in the Helena National Forest in Lewis and Clark County.  The Continental Divide Trail traverses north and south through the pass.  At the time of the Lewis and Clark Expedition the pass was a much-used pathway where the native people living in what today is Montana crossed over the continental divide.  The pass was crossed by Meriwether Lewis on July 7, 1806, on the return leg of the Lewis and Clark Expedition with a party of nine men and his dog Seaman.  The pass was named for the expedition's two leaders–Lewis and William Clark.  Lewis and Clark Pass is the only roadless pass on the entire Lewis and Clark National Historic Trail.  It has gone from being one of the most used continental divide passes prior to the pioneer era to one of the least visited passes today.  It can be accessed by a 1.5 mi trail.  Visitors will encounter the pass much as Lewis did in 1806.  The furrows left by the countless dog and horse travois that crossed the pass are still visible (though fading) and this is one of the places along the expedition's route that visitors may still encounter a grizzly bear.  On a clear day, like Meriwether Lewis in 1806 one can see Square Butte in Cascade County, Montana, 40 mi to the northeast. Elk Park Pass, elevation 6352 ft , is a mountain pass on the Continental Divide in southwestern Montana, about 4 mi northeast of Butte.  It is traversed by Interstate 15 and is one of three crossings of the Divide by the Interstate (all in Montana), the others being Monida Pass (on the border with Idaho) and Deer Lodge Pass southwest of Butte.  Elk Park Pass lies on the border between Silver Bow and Jefferson counties and is remarkable for its highly asymmetrical nature.  The approach from the north is through namesake Elk Park, a high, mostly treeless plain (like South Park in Colorado or the Sierra Valley in California), and the grade is almost imperceptible.  The elevation at the northeast end of the park, about 10 mi from the pass, is about 6200 ft , only 152 ft lower than the pass itself.  However, at the south end, the highway drops 750 ft from the pass into Butte over a distance of only about 4 mi . Milner Pass, elevation 10759 ft is a mountain pass in the Rocky Mountains of northern Colorado in the United States.  It is located on the continental divide in the Front Range, within Rocky Mountain National Park, along the boundary between Larimer and Grand counties.  The pass provides the passage over the continental divide for US 34, also known as Trail Ridge Road between Estes Park and Grand Lake.  The pass is not, however, the high point on Trail Ridge Road, which crests at 12183 ft east of the pass within Rocky Mountain National Park.  Along with the rest of Trail Ridge Road, the pass is generally closed in winter from the first heavy snow fall (usually October) until the opening of the road around Memorial Day.  The gentle pass divides the headwaters of the Cache la Poudre River (which issues from Poudre Lake just east of the pass) and several creeks near the headwaters of the Colorado River to the west.  The road near the pass provides a panoramic view of the Never Summer Mountains to the west.
Answer: between Jefferson County and Silver Bow County