Question: Which midsize car  of 1973 to 2005 used a shared platform with Pontiac 6000?

Context paragraphs:
Ford CDW27 platform: The Ford CDW27 platform was Ford's midsize car automobile platform from 1993 to 2006, It was co-designed by Ford and Mazda and was designed to be used as its 'World Car' platform.  The platform was developed over six years and at a huge expense totalling $6 billion, but was expected to save 25% compared to developing separate models for Europe and North America The design is based on Mazda's GE platform, used by the Mazda Cronos/626

Ford CD3 platform: The Ford CD3 platform (for "C/D-class") is a Ford midsize car automobile platform.  It was designed by Mazda.  The original Mazda chassis code for the CD3 platform is Mazda G platform, which Ford Motor Company has used every evolution of since 1983.  The platform is designed for either front, all-wheel drive or Ford's hybrid powertrain.

Pontiac G6: The Pontiac G6 was a midsize car that was produced by General Motors.  It was introduced in 2004 for the 2005 model year to replace the Grand Am.  The car was built on the GM Epsilon platform which it shared with the Chevrolet Malibu and Saab 9-3 along with other General Motors vehicles.  Features included a remote starting system (standard on GT, optional on base model), traction control/ABS, electronic stability control, automatic headlights as well as a panoramic sunroof option.  Production ended in 2010.

Pontiac 6000: The Pontiac 6000 is a mid-size car that was introduced by Pontiac in 1981 for the 1982 model year, positioned between the smaller Phoenix and larger Bonneville (previously the Le Mans).  It shared its platform with the Buick Century, Chevrolet Celebrity, and Oldsmobile Cutlass Ciera.

Buick Century: Buick Century is the model name used by Buick for a line of upscale performance cars from 1936 to 1942 and 1954 to 1958, and from 1973 to 2005 for a mid-size car.

Ford CD4 platform: The Ford CD4 platform (for "C/D-class") is a Ford global midsize car automobile platform.  The platform is designed for either front or all-wheel drive and Ford's hybrid powertrain.

Pontiac Grand Am: The Pontiac Grand Am was a mid-size car and later a compact car produced by Pontiac.  The Grand Am had two separate 3-year runs in the 1970s: from 1973 to 1975, and again from 1978 to 1980.  It was based on the GM A platform.  Production of the Grand Am was canceled in 1980 when it was replaced by the Pontiac 6000.  The Grand Am was reintroduced in 1985 when it replaced the Pontiac Phoenix.  It was Pontiac's best selling car and later replaced by the Pontiac G6, so named as it was intended to be the 6th generation of the Grand Am.

Ford EUCD platform: The Ford EUCD platform (for "European D-class") is Ford's global midsize car automobile platform launched in 2006.  It is a stretch of the compact Ford/Volvo/Mazda-designed Ford C1 platform.  EUCD and C1 share many suspension, brake, and electrical systems.

Pontiac Can Am: The Pontiac Can Am is a midsize muscle car built by Pontiac and based on the Pontiac LeMans and the Pontiac Grand Am.  The Can Am was a special edition option package and was only available in 1977.  It was named for the Can Am racing series, continuing the race theme used for the Pontiac Grand Prix, LeMans and Trans Am.

Oldsmobile Cutlass Ciera: The Oldsmobile Cutlass Ciera is a mid-size car that was sold from 1982 through 1996 by Oldsmobile.  It shared the front-wheel drive A platform with the similar Buick Century, Pontiac 6000 and Chevrolet Celebrity; both Oldsmobile and Chevrolet considered using the Celebrity name, which had originally been used by Oldsmobile in the 1960s.  Available body styles included a 2-door coupe, 4-door sedan, and the Cutlass (Ciera) Cruiser station wagon.

Supporting paragraphs: Pontiac 6000 Buick Century
Answer: Buick Century