Testament

The Wetherly familyâhusband Tom (William Devane), wife Carol (Jane Alexander), and children Brad (Ross Harris), Mary Liz (Roxana Zal), and Scottie (Lukas Haas) -- live in the fictional suburb of Hamelin, California, within a 90-minute drive of San Francisco, where Tom works.
On a routine afternoon, Carol (a stay-at-home mom and volunteer for school functions such as directing the school play) listens to an answering-machine message from Tom saying he's on his way home for dinner. Scottie watches Sesame Street on TV as Brad adjusts the TV antenna on the roof, when the show is suddenly replaced by white noise; suddenly, a San Francisco news anchor appears onscreen, saying they have lost their New York signal and there were explosions of "nuclear devices there in New York, and up and down the East Coast." The anchorman is cut off by the Emergency Broadcast System tone with the Civil Defense Insignia being shown on the Screen, then an announcer's voice states that the White House is interrupting the program: "Please stand by for the President and do not use the telephone." At the introduction of the President of the United States (who is never seen; only the Presidential Seal is shown on the TV), the phone rings but it goes dead along with the TV and electricity just as Carol answers it. Suddenly, the blinding flash of a nuclear detonation is then seen through the window. The family huddles on the floor in panic as the town's air-raid sirens go off; minutes later, several of their neighbors are running around on the street outside, dazed in fear and confusion. The family hopes Tom will return, but the circumstances are hard to ignore.
The suburb of Hamelin survives relatively unscathed, because apparently the town is far enough from San Francisco to avoid blast damage. Frightened residents meet at the home of Henry Abhart (Leon Ames), an elderly ham radio operator. He has made contact with survivors in rural areas and internationally, and tells Carol that he was unable to reach anyone east of Keokuk, Iowa; a radio report told of a conjectured errant bomb hitting Yosemite National Park, causing trees and rocks to fall from the sky like rain. He reveals that the entire Bay Area and all major U.S. cities are radio-silent. The morning after the attack, they are joined by a child named Larry (Mico Olmos) who tells Carol his parents never returned home from San Francisco (he was alone all night); he is soon part of the family, but later succumbs to radiation poisoning. Despite Abhart's efforts, no one knows or finds out the reason for the attack nor the responsible parties. Rumors from other radio operators range from a Soviet preemptive strike to terrorism.
The school play about the Pied Piper of Hamelin was in rehearsal before the bombings; desperate to recapture some normality, the town decides to go on with the show anyway. The parents smile and clap, but their smiles are forced. Hamelin escaped bomb damage, but not the significant radiation from nuclear fallout. The day after the attack, the children notice "sand" on their breakfast plates: contaminated fallout dirt settling back onto the ground from the blast. Residents have to cope with losing municipal services, food and gas shortages and, ultimately, the loss of loved ones to radiation sickness. Scottie, the first to succumb, is buried in the back yard. Carol screams at a Catholic priest (Philip Anglim) that she will not bury Scottie without his favorite (and missing) teddy bear. Wooden caskets are used as fuel for funeral pyres instead as the dead accumulate faster than they can be buried. Carol sews together a burial shroud out of bed sheets for her daughter, Mary Liz, who also dies from radiation exposure.
While many of the children die, older residents fall to rapid dementia, and order in the town starts to break down as police and firefighter ranks dwindle from the radiation. A young couple (Kevin Costner and Rebecca De Mornay) leave town after losing their infant, hoping to find safety and solace elsewhere. Carol's search for a battery causes her to listen once more to her husband's final message on the answering machine. To her sorrow, she finds a later (and previously unheard) message on the machine from Tom: he decided to stay at work late in San Francisco on the day of the attack, and she now gives up her last hope that he will someday return home.
Son Brad, forced into early adulthood, helps his mother and takes over the radio for Henry Abhart, who eventually dies. A bully who tormented Brad is caught breaking into their home; Brad tries to fight him off, but Carol scares him away. He manages to steal Brad's bicycle, and Brad starts using his father's bike, symbolically becoming the man of the house. The family adopts a mentally disabled boy named Hiroshi (Gerry Murillo), whom Tom used to take fishing along with the other Wetherly kids, after Hiroshi's father Mike (Mako Iwamatsu) dies.
One night, Carol is outside when she sees a pile of bodies being burned. Stopping and staring at the fire for a moment, she then breaks down and cries. Carol decides she, Brad and Hiroshi should avoid a slow and painful death by radiation poisoning and instead take their own lives via carbon monoxide poisoning. They are all sitting in the family's station wagon with the engine running and the garage door closed, but Carol cannot bring herself to go through with the deed. The three end up sitting by candlelight to celebrate Brad's birthday, using a graham cracker in place of a cake. When asked what they should wish for, Carol answers: "That we remember it all...the good and the awful." She blows out the candle. In closing, an old family film of a surprise birthday party for Tom plays, showing him as he blows out the candles on his cake.

Question: How does Carol decide they should take their own lives?
Answer: Carbon monoxide poisoning