123. Read the following passage, then answer the question below.

In April 1815, Mount Tambora on the Indonesian island of Sumbawa erupted in the largest volcanic event in recorded history. The explosion was heard more than two thousand kilometers away, and ash fell over a vast area of Southeast Asia. Tens of thousands of people died in the immediate aftermath from pyroclastic flows, tsunamis, and famine, and many more perished in the months that followed as crops failed across the region.

The eruption's effects were not confined to Indonesia. Besides ash, the volcano injected enormous quantities of sulfur dioxide into the stratosphere, where it combined with water vapor to form a haze of tiny droplets. Because these droplets reflect sunlight, they cooled the planet's surface. Average global temperatures are estimated to have fallen by roughly half a degree Celsius, which sounds modest but was enough to disrupt weather patterns. The following year, 1816, became known in Europe and North America as the year without a summer. Frost struck New England in June and July, and in parts of Europe relentless rain ruined harvests and drove up the price of grain. Hunger led to unrest and, in some regions, to migration.

Historians continue to debate how much of the misery of 1816 can be blamed on Tambora alone. The years before the eruption were already cool, partly because of a lull in solar activity and an earlier, unidentified eruption in 1809. Nevertheless, the episode has become a reference point for scientists who study how volcanic particles affect climate. Their research suggests that the cooling is temporary, usually lasting only a few years, because the droplets gradually settle out of the atmosphere. Some researchers also use Tambora as a rough guide to what might follow a similarly large eruption today, when a disruption could spread quickly through interconnected food markets.

The word 'relentless' in paragraph 2 is closest in meaning to